The Trainer #67 - When Fuel Trims Won't Trim

In the May edition of “The Trainer” we explored how the ECM maintains fuel control. We also talked a bit about the factors that can cause fuel control to get out of whack.
This month, we focus on what to do when it does get out of whack. System Lean DTCs are among the top 10 Diagnostic Trouble Codes technicians face in the shop and can be a challenge to correct. System Rich codes, while not quite as common, can often be even more difficult to isolate. It’s not hard to understand why the Engine Control Module sets these codes. That, of itself, is pretty simple. If Total Fuel Trim numbers exceed a programmed threshold, plus or minus, the ECM will turn on the Check Engine light. It’s the reason the trim drops or skyrockets that we have to figure out.
Most of us start by looking at the amount of air getting in and the amount of fuel being added, don’t we? After all, if it’s too “lean”, it must be too much air or too little fuel. The opposite is true for a “rich” condition, or so you would think. What if the code is set for only one bank? What then?
The ECM has to know how much air (by weight) is coming in and then calculates the amount of fuel (by weight) to add. It doesn’t do this intuitively - it has to be told, either by inputs from a variety of sensors or by its internal programming. Sensors that are misreporting may make the ECM “think” that the mixture is rich or lean, when it really isn’t. Even the software may be incorrect, at least under certain operating conditions, and a reflash may be the fix.
We’ll cover all of those variables and talk about efficient ways to check for, and isolate, the REAL cause of these DTCs. Be sure to watch!
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Due to factors beyond the control of Motor Age, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. Motor Age magazine (UBM Americas - Automotive Group) assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. Motor Age recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video. Due to factors beyond the control of Motor Age, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not Motor Age magazine or its corporate parent, UBM Americas.

[@saulorozco5787]

Even i am not a native english speaker i understand you thank you great video, you only open your mouth and i can see your experience thanks.

[@sollykhan2385]

EXCELLENT VIDEO, Thanks once again Pete, your a LEGEND 😇

[@punditgi]

Amazingly helpful videos every technician should see and learn from.

[@MotorAgeMagazine]

Thank you! I'm glad you found it helpful.

[@Mike-pb7jm]

This is one of the best youtube videos I've seen, thank you for making these. Coming from a chemist, the process of separating the oil/water in that last clip of the video is called an extraction. Extracting alcohol from the fuel into water.

[@shawnfanning8519]

Well explain, you got the best video on fuel trim, thank alot

[@davidm3210]

Great tutorial, thank you sir.

[@mike-yp1uk]

I like the going back to basics.

[@romanjaspe4172]

Thanks so much teacher. Regards 👍

[@daco8252]

I have a 98 5.7 chevy, code P0171 Bank 1, check light comes on and goes off occasionally, first thought culprit was mass air cover gasket over trottle body. After a code reset I can drive 40 miles most of the time before light will come back on, but not always. At idle st runs 0.0 - 0.8 to 1.6 constantly lt around 19 to 23.4. I have smoked, sprayed and propane around everything, no leaks found in unmetered air. Do you think I have a lazy bank one up stream o2 sensor? I also have cleaned map and maf sensors. Motor idles and runs good Can you offer any help please?. Thanks!!!

[@coycarlson4979]

Once AGAIN - GREAT INSTRUCTION 👍👍👍👍👍😊😊😊😊😊😊✌️✌️✌️✌️

[@MotorAgeMagazine]

Thank you Coy.

[@johnrewijk8371]

I'm driving in Europe ( Netherlands) that is. My Mitsubishi is a flex fuel car . Regular gas to start it up and liqified petroleum gas to run . L.P.G offers less energy compared to regular fuel. Does this make long term fuel trim go up ? My simple scan tool shows lower trims on regular gas. Like to learn about this , Thanks for teaching so well ! Greetz John.

[@dannyred5710]

Outstanding thank you

[@alvinmcfarlane7238]

Nice video

[@Ryan-J]

Hey amazing video with great detail however im at a loss car was throwing code p0100 maf sensor replaced it engine ran so much smoother no more rough idle but now i get code p0170 fueltrim bank 1 astra h 1.8 2006 LTFT(B1) has a constant 11.7 o2s1 (B1) fluctuate between 0 and negative 25 o2s2 (B1) says negative 100 and doesn’t move any idea where to start i was thinking maybe o2 sensor is bad but im no car expert would be great full for any advice thanks in advance

[@padraiggalvin2807]

Thank you for sharing this good information. You'd make a great teacher

[@eriginez5171]

Steve if it has a map sensor and maf sensor is it still a speed density engine ..

[@Xantylon74]

One of our vehicles, a 2005 VW Beetle, 1.6L with a widedband oxygen sensor is running rich. At idle it's -2.5 at 2500-3000rpm it's -10 to -20. The long-term fuel trims don't adjust the short term, but they aren't as bad as the short-term ones, which are always around -1.6 to 3.5. The pre-cat lamba probe is switched off quite quickly by the ECU while driving and the MIL comes on a little later. Error code P0130 The pre-cat lambda sensor was changed, but it hasn't changed. The exhaust manifold is tight, I couldn't measure the fuel pressure yet. The probe after the cat (normal o2sensor) has up to 800mv in operation. The voltage from the upstream o2 is about 1.5 V according to the scan tool, but Voltage reading on a wideband O2 should not say anything. The heaters from the sensors are working according to the scan tool. What would you guess?

[@bryang2724]

I have what I think is strange. My 01 ford explorer 4.0 sohc set both p0171 and p0174 a couple weeks ago. I looked at the short and long fuel trims the other day and both banks were +10 on short without fluctuating and -9 on long without fluctuating. Idle seems normal and drivability seems normal. I can't afford a shop so it's taking longer to diagnose. Both bank 1 sensors had been moving between .1 and .7. it's been cold and rainy with no garage which makes it harder for me to take time.

[@robertbumbalough5469]

What data other than Mass Air Flow is used by a vehicle PCM to calculate the amount of oxygen that should be in the exhaust which is then used in comparison with the oxygen sensor data to enable the PCM to calculate fuel trim?

[@MotorAgeMagazine]

Robert - I think Bernie Thompson explains it better than I can in a recent article he wrote for us. Read it here: www.searchautoparts.com/motorage/electrical/science-mass-airflow-maf I would clarify one thing. The oxygen sensor does not provide an input that the ECM uses to calculate initial pulse width (on time) for the fuel injectors. It only provides feedback on how well the ECM hit the mark. Based on that, the ECM modifies the on time and stores the difference (for that load and rpm) as Long Term Fuel Trim. Hope this helps!

[@James-fs4rn]

👍thank you!

[@bricekoirokpi9954]

Thank you Sir 🙏

[@jimmynitro9248]

First off thats one excellent video you got there sir. Just one question The other day i had a lean code for one bank and saw there was an exhaust leak right before the up stream o2 sensor, Well i got that fix and now i got the car is running rich only during acceleration longtrim -10 to -14 and shorttrim bouncing from -5 to -9 when i step the foot off the gas pedal or makes a stop the the trims are okay. Is there anything i should look at or do the trims adjust overtime after fixing the leak.? Thank in advance

[@MotorAgeMagazine]

You may be looking at a correction now that you fixed the original problem. Drive it a few days and see what happens.

[@profitnessstore]

Great explanation! I hope you might be able to shed some light on a condition i have. 2012 GMC Acadia. It runs perfect for hours then i get a strange condition on bank 2. The STFT and LTFT both go to -100% without moving, and they had been running normally at about 5-7% LTFT and STFT going aroun -5 to +5%, then it just goes to -100%. When it does this i get random misfires at idle and throughout rev range. I reset the fuel trim learn and it goes back to running perfectly for hours. I have replaced several parts MAP, MAF, bank 2 sensor 1 O2, intake manifold gasket, pcv and hose, spark plugs. Nothing seems to make a difference. I have checked fuel pressure and it bounces between 52-60 at idle, and fuel rail pressure around 400-500 at idle. Any ideas?

[@MotorAgeMagazine]

Any codes (and related freeze frame info) to share?

[@markpoposki]

Hello sir, love your videos and have been a sub since day 1. Had a question for you. I have a bmw z3 with lean codes. Car has extensive service history. Had it for 12 years. To diagnose the problem i smoked it, no leaks. I watched your video on fuel trims and my situation is very odd. Trouble codes are p071 p074-lean bank 1 and 2. Short term FT are -6/-7 for bank 1 and 2 Long term fuel trims are 11.7 for both banks. And they stay 11.7 with rpm at 2.5k. Interesting thing is short term fuel trims go up to + 7 with rpms going up to 2.5k O2 sensors. I’m getting normal reading for both bank 1 pre and post cat sensors. Bank 2 o2 sensors stay at .8 value. I’m really confused. I did order maf cleaner and fuel pressure tester. What should I check next. What do you think is the culprit here? Thank you!

[@MotorAgeMagazine]

Have you tried resetting/clearing the codes? If so, do they return right away? The only thing that stands out is the weak upstream O2 sensor. But I would suspect a related code for one that is "stuck" at any value.

[@robykirbyify]

Hi, I understand fuel trims and thankfully a little more after the video, but I have a question. Q. my LTFT is constantly high at 36% and above, this does not trigger any codes or engine light. The puzzling thing is that if I put the electrics under load by having the lights on full, operating the cooling fan or have the windows going up or down and then holding the button down. The fuel trims drop to around the 0% -/+ 5%. What is this about. Thanks.

[@MotorAgeMagazine]

That is very strange. Typically, when strange things happen related to electrical components, I look at the primary grounds. If there is a voltage drop issue on the ground side, perhaps it is affecting the O2 sensor's output. Try watching it on your scan tool in graphing mode while putting the load on. Another potential cause that comes to mind is the scan tool itself - not knowing which you are using to access this data. Finally, what is STFT doing all this time?

[@mharradine57]

Thanks, Pete. How would you address a lean condition where while giving 2500rpm in park the stft goes to 0 and the ltft only improves from say 12 to 11. I lost FF data but something made it throw the p171 and p174. I can't reproduce it on test drives.

[@rassielcoto9385]

if stft was positive at idle and at 2500 rpm it goes to 0 that is a vacuum or air leak

[@johnmckamy6398]

you wont see this on a test drive because the amount of air entering the engine at road speed is overcoming your vacuum leak. the vacuum leak is having the most effect at idle . your computer is only adding a certain amount of fuel for the air it can account for. it cant calculate the amount of air entering the engine through the vacuum leak because its bypassing the maf sensor . I would check the intake gaskets with some water or get somebody to smoke test the engine when its cold.

[@LAQueenBee0313]

Struggling with my LTFT on a 18 Mustang Eco. It’s at a +15% at idle (or when letting off gas) Changed Fuel Pressure Sensor aswell as gasket before down pipe (but it doesn’t fit snug for some reason, it’s snug enough but can move when pressure is applies) could it be Injectors? Only code I’m getting is P0420 (I was told it’s because of the Aftermarket down pipe that came with the car)

[@richard1835]

Thank you

[@iam1smiley1]

Thanks! I really like your alcohol/water test :) I recently got a new scan tool and it says the gasoline in my flex fuel 08 Caravan 3.3L has 25% alcohol and I was surprised to see it that high, as I thought up to 10% was normal in gasoline. Does 25% seem like a rather extreme calculation to you? Currently having gasoline shortage in Canada due to war/environmentalism from governments....are they selling me extra alcohol or should I look further?🍸

[@MotorAgeMagazine]

Definitely sounds off. Sometimes the ECM miscalculates and that causes fuel trim codes to set. Measure the actual alcohol content of the fuel in the vehicle and start there.

[@pitchforkpeasant6219]

Have an update to your findings?

[@rydgomez2]

I got a unrelated topic question. I couldn’t find a water pump video from you. I have a question about Joanna’s Durango. Last month it was in there for an oil change and you guys found a coolant leak. I checked it out and found the water pump was leaking at the shaft. Easy job so I replaced it myself, but a week later we smelled coolant again and last night I finally got around to looking for it and turns out it’s leaking again at the water pump shaft, I looked all around to make sure it wasn’t coming from anywhere else like the oil cooler or hose and it isn’t. It starts at the water pump pulley on the inside. It doesn’t start leaking right away but until after about 10 minutes of running any idea what could be causing it to do that ? My initial guess is that there is to much pressure building up in the system. I hope it isn’t any major. It’s never over heated and as you know we take it in for oil changes all the time. if you don’t mind giving me some input I’d appreciate it. 2016 Durango 3.6 liter. 113k. Thank you.

[@MotorAgeMagazine]

Good afternoon Ray! First, we didn't change anything - we're a trade publication, not a shop. But to answer your question, it is important to lubricate the seal prior to start up or it can fail prematurely. That could explain your situation. And we do have a few videos on water pumps: kzbin.info/www/bejne/ioGVpJuogd2Ugrc kzbin.info/www/bejne/pXfKi6d3jqyYnas

[@sk8n854]

What could cause a p0171 on a v8. Bank 2 looks good at all times. Bank one looks good under idle. But adds a lot of trim under load. Got the code driving in an area with steep hills. The injectors are all fairly new oem injectors. 150k mile 2007 mustang gt. MAF is clean. Oxygen sensors seem to be working fine as far as i can tell.

[@katrosper]

thank you

[@PontiacSpazz]

I’m having a problem. My short term is running maybe from 10-16% on bank 1 more than bank 2. I don’t have no vacuum leaks and I checked injectors for leaks and found nothing. What could it be

[@ysnphotos]

Amazing 😉

[@besofacacia6103]

Mr. would you plz answer my question ? Is is normal to loose long term fuel trims on every cold start ? BMW 2002 m54.

[@MotorAgeMagazine]

If you mean that the LTFT is resetting to 0, then no - that's not normal. I would check the battery. If the voltage is low, the ECM and other ECUs can't maintain memory on initial start up.

[@charliereynolds4593]

Does the 2015 Chevrolet Silverado 5.3l have a place to test fuel pressure

[@bradansell8715]

I have an 03 Murano, Fuel trim 2 is open, getting random misfire on #3 cylinder, question, should fuel trim 2 be closed? is it related to the random misfire?

[@MotorAgeMagazine]

I think you're saying that Bank 2 is in "open loop" - meaning that a condition exists that will not allow the ECM to effectively manage fuel. It could be related to the random misfire, so tackle that first. What does Freeze Frame have recorded?

[@3b0ny1]

What is happening when the long term fuel trim gets more lean as the RPM goes up?

[@MotorAgeMagazine]

On a MAF equipped vehicle, when fuel trims trend lean across the rpm range (on both banks of a "V" engine) look at the inputs to the ECM, especially the MAF sensor.

[@ysnphotos]

Nice 👍🏻

[@jordan_llm]

Hey Sr.! My 2009 Kia Sportage 2.0 16v (MAP) has a rough idle with high LTFT which stays at 16.41% regardless of engine RPM. STFT bounces to +/- 5% during rev ups (negative when increase and positive when decrease RPM) and LTFT stays on 16.41%. The O2 Sensors 1 its ok, on 0.1 and 0.9 range, It is causing high fuel consumption. Any tips as to what's going on?

[@wecarespares]

Do you have CEL and error codes?

[@milivojnesic9013]

Thanks for great explanation. I have problem with my vw polo 9N (AZQ engine), my fuel trim isn't triming - LTFT is constant at 0% while STFT is fluctuate depending on condition. At idle STFTis about -20% (LTFT constantly 0%). Pretty similar situation is in driving condition. I have P0172 code (no vacuum leak, checked by smoke mashine, but occasionally I get P0106 code). Did you had this case? Thanks in advance.

[@MotorAgeMagazine]

Post your fuel trims (short and long) for idle and 2500 rpm.

[@milivojnesic9013]

@@MotorAgeMagazineThank you for replying. Here you can see live data: kzbin.info/www/bejne/p3O1dmlorZyMgLs

[@MotorAgeMagazine]

@@milivojnesic9013 Your STFT is trying to correct for the "rich" condition but your LTFT is at "0". Have you reset the ECU memory recently? I also don't like the way the upstream O2 is stuck high and there is little switching in the STFT. Test the front O2 sensor.

[@milivojnesic9013]

@@MotorAgeMagazine in this condition i drowe ~500km, i haven't reset ECU since. I also changed front O2 sensor from good vehicle and it is same situation

[@milivojnesic9013]

@@MotorAgeMagazine Here are same data with different O2 sensor: kzbin.info/www/bejne/iaXZeqGvapWUoKc In very rare situation LTFT (under 10% of using the car) fluctuates. Thank you for your time.

[@rydgomez2]

Do you got an email so I send you some fuel trims. ?

[@sleepyheadp1926]

Wish I could add photos to comments . My data also seems wonky on temps n voltages . IAT reading .56F ECT 30.56f

[@jasonashley9853]

My STFT is about 20% lean at idle at 800 RPM, it goes back towards stoich under throttle, but if it drops below 800 RPM, it also goes back towards stoich. I'm custom tuning the car, it's a 16 focus. My tuner thinks the heating element is going out in the 02 sensor. What are your thoughts on this?

[@MotorAgeMagazine]

What is the fuel mode under 800 rpm? I'm thinking it's going back into open loop.

[@jasonashley9853]

@@MotorAgeMagazine fuel mode under 800 RPM? What exactly do you mean? Also, the old 02 sensor was pretty black. Long term trims are running rich at 0.94.

[@MotorAgeMagazine]

@@jasonashley9853 There should be a PID on your scan tool that tells you if the ECM is in "Closed Loop" or not.

[@eis-bl4xw]

My car is a turbo i-6 3.0L. My idle ltft is 23%, stft varies but +/- 10% but mostly around 0, my maf reads 3.06g/s OR 3.4g/s. I am not sure which conditions the MAF reading changes but Im sure one of them is from a cold start and the other is on a warm start. I sometimes get "rail pressure too high before start" code causing limp mode that goes away when I restart the car. My lean code says it happens on partial load. WOT from 2.5k rpm to 7k rpm, ltft stays, stft slowly drops to -10%. Whats going on? Ive cleaned the MAF, swapped rail pressure sensor, reset adaptations. At this point, Ill consider anything reasonably priced.

[@MotorAgeMagazine]

Post your fuel trims at warm idle and at 2500 rpm.

[@Romeo-yq3vf]

My 2013 Ford Escape titanium has system lean and cylinder 3 misfire codes. Running rough at idle. I believe it’s a MAP system with one sensor just passed the air box(replaced this one), and the other is in front the motor on the boost manifold. LTFT steady at +27% and stft is +\- 5% regardless of rpm. Any ideas of what’s happening here??

[@MotorAgeMagazine]

The first problem to tackle is the misfire. I suspect that the injector is not delivering fuel to that cylinder. Here's why... The engine is taking in the same amount of air so the ECM commands the injectors to deliver the correct amount of fuel. But one cylinder isn't - meaning the engine is actually getting 25% less fuel for the air ingested than it should. (I'm betting it's a four-cylinder). That results in the lean condition you're getting the P0171 for. First place I'd look anyway...

[@Romeo-yq3vf]

Ok I will sure focus more on the fuel delivery. thank u sir 👍

[@tylerfochtman1578]

So... Very informative video... Have been trying to track down the culprit of my phantom lean condition that only seems to come about when the ac compressor fan kicks on and there is a higher load on the engine. Makes me think there may be some fuel delivery issue or some odd vacuum leak from the fan or possibly an electrical issue that disrupts the proper calculations by the ECU. I'm not totally sure, but was looking for a bit more insight. I just patched my exhaust and now it only goes lean after warm up on bank 1.

[@MotorAgeMagazine]

Need some baseline info - YR, Make, Model, Engine, mileage? Are there any codes? What are the fuel trims (STFT and LTFT) at idle and when the problem occurs?

[@tylerfochtman1578]

Sorry. Thanks for the response. It is a 2005 Nissan Pathfinder se-off road. It has 133k miles. Brand new catalytic converters (4) just inspected in may. Noticed a leaking exhaust sound and had the p0171 code for the lean bank 1 (passenger seat side of engine) this code cleared after patching some of the leaking joints, but now the fuel trims are around 0 at idle ( fluctuating around +/- 5%) but if I add a load on to the engine, say I max out the ac, the short term bank 1 starts to climb up, if I am driving and the condenser fan kicks on, shortly after I watched the short term bank one only rise up to 25% and stay there, unless opening the throttle significantly. The other bank seems to be fine. I'm wondering if maybe an o2 sensor may be giving bad data when it gets hot or there is some electrical issue or stored memory to add fuel on that side when there is a certain load added. Not too sure. Although I am worried about the fact that I replaced the fuel delivery hose, but thought if this was the issue, there would be a more pronounced issue among both banks, but that might just be my mind not wanting to admit I may have screw up the fuel delivery pressure or volume as you mentioned, when the load is added and this disrupts the fuel volume and since it adds to the one bank, maybe it's enough and doesn't need to mess with the other side, not too sure. I might unplug the fan and see if that may be a source of some shortage or something. I found a few popped fuzes, 2 for the transmission in the engine bay and one in the clove box for the lighter port. The mechanic I went to said that there is probably some electrical issue with my cluster since my oil pressure light is on, and charged me 200 to swap out the sensor without asking lol,. I feel like these issues may be feeding into each other. Heard that this engine has problems with the timing chain tensioners wearing out which causes excessive heat which then blows out the oil gallery gasket in the rear timing chain which causes my lack of oil pressure which may cause more of a load on my engine (~23% @ idle, not sure what it is supposed to be) which causes the fan to kick on more often causing some sort of electrical short which for some reason indicates to my engine to add more fuel to speed it up a bit to account for the load and the lower oil pressure. Idk. I'm trying to piece it together from my limited experience from watching helpful experts like you who have graciously given their time to help others. Figured I'd ask someone with extensive knowledge willing to give me an idea and would be happy to send a tip for a tip to your PayPal or something

[@tylerfochtman1578]

It's a 4.0 L V6. Still causing issues. Just ordered a new exhaust and tail pipe with hardware and cement to try to seal it the best I can and eliminate any exhaust leaks disrupting my sensor data. It seems like there is a load switch or something that gets turned on and causes fuel to be added on the bank one. I watched the bank 1 short term trim go from evening out (-9) to 25% positive. The O2 sensors seem to be out of sync as well, with the bank 2 at 600 and the bank one at 300 at idle. Maybe when the computer starts accepting oxygen sensor data, this causes the lean condition. Just don't want to be shelling out for O2 sensors if I dont have to. Might put my old ones on and see what it reads. Try to get all NTK or whatever is the OEM manufacturer so hopefully they'll all be on the same page at least with baselines and what not, I hope. Other than that, I have that squealing sound like an ac clutch or something, but it sounds like it's coming from the bank 2 (driver side) valve cover, and it is known to have timing chain tensioner failure due to the improper manufacturing of the timing chain teeth, being too sharp for the plastic tensioners. Requires a complete timing overhaul, I think this issue causes an increase in internal temperature which causes the rear timing cover gasket (oil gallery gasket) to fail prematurely resulting in low oil pressure after warm up. I'm wondering how these issues could exacerbate each other or be the result of one or the other, and hopefully get to the bottom of it. The electronic failures seem to be most troubling. Might want to rule out any extensive goose chase with the ECU and wiring before I throw any more time and money into it.

[@MotorAgeMagazine]

@@tylerfochtman1578 You never mention LTFT. We need to consider the overall total trim, so please - so that I can try to help - record the total fuel trim at idle, then while held at a steady 2500 rpm. Fluctuations in speed will create short term fluctuations in trim as the ECM catches up to the changes and since i can't view the data personally, I can't tell if that is part of the problem.

[@tylerfochtman1578]

kzbin.info/www/bejne/lWe4kqGjbbOrm6c

[@thromboid]

In a speed-density system, if there is a vacuum leak, is it possible that the idle air control system could compensate and still give a normal idle speed?

[@MotorAgeMagazine]

Typically, no. Look at the IAC position on a scan tool. If it's close to or at "0", there's a vacuum leak.

[@thromboid]

@@MotorAgeMagazine Thank you - this particular vehicle has very basic OBD and doesn't report an IAC PID, but I can probably scope it directly. Really appreciate your thorough and professional video presentations!

[@mugsymydog]

Pete, Thanks for your videos... I am working on a 2005 Hyundai Sante Fe that came in with Lean fuel trim on both banks. I located tear in intake boot and replaced it. I found MAF sensor was giving erroneous readings and replaced it, I then found purge solenoid was stuck open and replaced it. Now I have good STFT at idle but as soon as i try to do drive cycle under a load LTFT ramps up and keeps commanding more fuel. I did smoke the system and do have smoke coming from valve cover but nowhere else. I put propane at these areas to see if I could see change in STFT but now change even at 2500 RPM. I guess my next test will be fuel pump but there is no schrader valve on this and I am not sure where or how to connect. I will be looking at fuel pump with Verus scope to see if I can see anything but there is absolutely no drivability issue. If you have any ideas let me know where to go. This is an E-check issue and not because of any other complaint. Thanks again for your time.

[@wecarespares]

Hi Have you found your problem? I have a Tucson with similar issue. But somehow My scan tool won't show STFT, the LTFT is constantly at 25. No matter at idle or driving. Apparently you need Hyundai special tool to check for fuel pressure at the fuel rail end, i made my own adapters and checked at where the supply hose connecting to the fuel tank. Pressure was normal. I did smoke test no leak. And i put propane in right before the maf sensor, the scanner is showing increase air flow and upstream o2 sensor started to read rich instead lean. I assume that means the MAF and O2 is working.

[@wecarespares]

Also i did check purge valve it's not stuck open

[@Paul1962]

Heres a interesting one for you, my Boxster 986 runs fine, no check lights, o2 readings fine, short term trims bob around zero, long term trims +6 and +7 no change under different engine loading or speed. its not a problem as the car runs fine, but I am always curios as to why the long term trims are high?

[@MotorAgeMagazine]

I really wouldn't consider that high. LTFT will increase with wear and tear. Like the old saying goes - If it ain't broke...

[@Paul1962]

@@MotorAgeMagazine well I agree with the ain't broken bit, lol

[@ramdodgetruck1177]

2003 impala 3.4 LTFT negative 25 engine at idle. STFT Positive 6. Cant find vacuum leak. Any tips what to look for? Enjoy your channel.

[@rassielcoto9385]

The vehicle is rich at idle it wont be a vacuum leak, look at the MAF pid at idle it should be around 3.4 g/s, if there are misfire codes check for leaking injectors a fuel contaminated oil is a good sign of a leaking injector along with misfires, check O2 sensors also

[@ramdodgetruck1177]

Rassiel, thanks for the reply. Will check it tomorrow. Looked at Maf sensor and it was spotless. there were no codes. Did oil change, no fuel smell.

[@eflanagan1921]

any update ?

[@ronvessells1356]

Yes the vehicle is rich at idle. The manifold vacuum is highest at idle as well. I would remove the vacuum hose from the fuel pressure regulator and check for fuel leaking into the engine from a leaky fuel pressure diaphragm. I have seen this on this exact vehicle and engine many times.

[@rondoschiavoni8840]

can fuel trims be adjusted mechanically on a Buick 3800 V6 engine 2003

[@MotorAgeMagazine]

No, they are not mechanically adjustable. Fuel trims are the displayed corrections made by the ECM's programming in response to a variety of sensors, depending on the system type - MAF or speed density. If the fuel trims are out of range, then you need to identify the cause and correct it to restore them to normal.

[@rondoschiavoni8840]

@@MotorAgeMagazine THANK YOU

[@mrprevost1]

Hi, I have a 2004 Silverado that’s running rich on bank 2 and only giving me a random misfire code my shrtft is 52.3 and lngtmft is 25. Can you help me please????

[@MotorAgeMagazine]

First, are you reading the data PIDs correctly? I've never seen fuel trims, STFT or LTFT, go much over 25-30. Second, what are the bank 1 trims doing? Under what conditions are you reading them? Post STFT and LTFT numbers collected at idle with engine at normal operating temperature and also at 2500 rpm in Park. We'll start there.

[@mrprevost1]

I’ll get to it tonight.

[@elcano033]

my car is throwing p0171 and p0174, I've replaced both upstream o2 sensors, maf sensor,egr valve, smoke the intake no leaks, when I check on my scanner for ltft it goes immediately to 25 on both banks after I start the engine, fuel pressure at 50 psi no injector leak down, the stft fluctuate all over the place!!!!! I have no idea what else to replace. 05 mustang 4.0

[@MotorAgeMagazine]

Edwin - Check the freeze frame data and take a look at the ECT PID. Is the engine warm or cold when the code sets? It is not unusual for the intake to leak when cold but not after the engine has warmed up. Also, check the fuel trims at idle and at 2500 rpm. If the trims move toward normal at the higher engine speed, then there is a leak - and this can be any source that is allowing unmetered air into the engine. Hope this helps!

[@elcano033]

Thank you for reply, the problem with the car was the programmer I had installed in the car, I'm calling diablo sport to see if they have a solution.

[@juanrodriguez-ry6yt]

it.s best to leave it all stock cold air kits chips headers all trash i'm old i can not stand to work on cars with all that crap on it

[@777jonah888]

post stft and ltft at idle, 1500rpm, 2500rpm, in park and then go drive and recreate the ft and post it all, ok

[@IntelligentElephant]

I'm at work and just happened to put my scanner on my truck. In the past fuel trims always looked normal. Today bank 2 is always staying at +2%. Idle and driving. I didn't check it before it closed loop. Could you tell me anything? Thanks your videos are awesome

[@MotorAgeMagazine]

Need to be a bit more specific - is this STFT or LTFT? Is it switching or locked in at 2%?

[@IntelligentElephant]

@@MotorAgeMagazine thank you for getting back to me. I've been binge watching your videos. It was short trim. But today it reads fine. Stays about -2 to +7, today. I did notice the Long stays -12 until I step on gas pedal. Then it goes straight to zero. Until I let off then goes back to -12 or close to it. Is that ok? I know 25 is usually the number that's not good. My truck is 2011 Silverado 5.3 (LMG). 240,000 miles THANKS again for these awesome videos. My electrical diagnoses is way better just in the last 24 hours from watching 2 or 3 if your videos. I also bought the xtool D8, it's new to market. Do you have any experience with Xtool?

[@MotorAgeMagazine]

@@IntelligentElephant Let's see if I can address your questions. First, it is normal for STFT to move back and forth across 0 - in fact, it HAS to for the oxygen sensor to work properly. Ideally, it should switch evenly on both sides. If not, the LTFT will learn the correction needed to bring it back into place. Now to your LTFT comment. This is a learned value. When you hit the throttle, it's possible you are going into Open Loop and the ECM isn't in fuel control OR that's the value learned for THAT load and rpm condition. I wouldn't worry about the -12 number - with 240,000 miles on it, the ECM is correcting for normal wear and tear. As for your comment about 25 being a bad number, that isn't entirely true. Every OE has its own TOTAL fuel trim threshold that must be crossed before it will set a System Lean or System Rich code. If the STFT is switching over 0, nice and even (within a few points), the ECM is happy and you can be to. Last, I know the name but I have no experience with the tool you mention. I'm glad you're finding these videos useful. That's why we do them!

[@IntelligentElephant]

@@MotorAgeMagazine it could be going into open loop when applying the gas? I never thought about that. So is that something I should be concerned about? To check that, I just need to have loop status pid up and see if it goes in and out when hitting the throttle? I promise I won't keep dragging this out, but learning things is just about all I do. Especially with this truck. My goal is half a million miles+.

[@MotorAgeMagazine]

@@IntelligentElephant You can monitor the PID but if it is going into Open Loop, it's because of the sudden change and nothing to worry about. If it never goes into Closed Loop, that's a different story!

[@harley8728]

Would a fuel trim greater than 5 or -5 cause a check engine light?

[@MotorAgeMagazine]

The ECM will turn on the MIL light for a fuel trim related code only if the TOTAL fuel trim exceeds the limits set by the manufacturer. Typically, LTFT will exceed +/- 25 and STFT will have to exceed +/-5 to 10 before a "system lean" or "system rich" code will be flagged.

[@harley8728]

@@MotorAgeMagazine TY

[@fusserick76]

What would cause one bank to go slightly lean positive 9 and the other to be at 0 at an idle? Map only v6 engine.

[@MotorAgeMagazine]

Is this TOTAl fuel trim? That's the number to be concerned with. If this is Long Term Fuel Trim, it's not what I would consider bad. If it's Short Term, it's in the high side of a normal range (I prefer swings from -5 - +5). If it's a total, it's a non-issue. Consider what is unique to that bank - perhaps an injector or injectors are dirty and not flowing the same amount of fuel?

[@fusserick76]

@@MotorAgeMagazine I appreciate your quick response. Those where long term numbers only. The short term at idle stay +/- 5. This is a personal vehicle and it has no cel. I'm just intestested in learning and preventing future issues. I will try some injector cleaner to see if that improves bank 1. Thank you.

[@gknettel22]

Oddly have a rich condition, but the plugs look lean. Getting 2 codes p0141 and p0138. Thinking pcm

[@robertpaul3675]

Hello sir I am having long term fuel trim at idle getting to up to about 14 most of the time my short term will be negative and getting up to 1 at times back and forth. Bank 2 long term getting up to about 10 short term negative too changed maf no vac leaks what do you think this could be this is during idle at higher rpms fuel trims go down what would you think this is?

[@MotorAgeMagazine]

Think about what you wrote - "this is during idle at higher rpms fuel trims go down". If trims are higher at idle and get better at higher rpms, you have unmetered air getting in somewhere.

[@robertpaul3675]

@@MotorAgeMagazine ok I have looked everywhere ill have to smoke machine it

[@fusserick76]

Any idea what would cause long term fuel trims on one bank to run positive 14. Then when put into reverse or drive it goes to 0. Instantly.

[@MotorAgeMagazine]

Long term, or LTFT, is a learned value stored in a table based on load and rpm. You should be.more concerned about total trim, or LTFT plus STFT. Compare that bank to bank and see what it looks like. Another key is to monitor Short Term (STFT). If it stays centered (+/-) around "0", the ECM is in fuel control.

[@fusserick76]

@@MotorAgeMagazine Thank you again for responding. I'm a newbie at diagnosing. The vehicle has a rough idle but runs very well. No cel. I cannot locate a vacuum leak. Fuel delivery is good. I dont want to parts change and waste money. The condition I described in the comment before the short term stayed at 0 +- a couple along with the long term. But in neutral at idle bank one stays at 14 total fuel trim. But in gear at idle it the engine smoothes out and fuel trims are all normal. The scan tool shows an instant change when selecting between gear position on the ltft. Stft stays around 0.

[@MotorAgeMagazine]

@@fusserick76 Post your vehicle info and I’ll try to off some ideas when I return Monday.

[@fusserick76]

@@MotorAgeMagazine 2011 dodge nitro 4.0L V6. Map only. 125000 miles. Rough idle in neutral. Some days it will have an extended cranking but it always fires up. Thank you for your help. I'll keep searching for the solution.

[@MotorAgeMagazine]

@@fusserick76 A fuel trim change that abrupt is unusual. I would clear the KAM (Keep Alive Memory) which will clear the learned LTFT values and see what happens. Monitor the trim on start up and see if STFT immediately starts to correct or you notice any change in idle quality. Let's start there...

[@jeffreyallen3777]

Very informative videos, thanks for taking the time, You explain things really well. I've got a 2009 f150 4.6 3valve that hasn't thrown a code yet buy has long term fuel trims that after warmup sit at around 25+ on both banks. They will drop to about half that under a load. I've pinched hoses, used carb cleaner and propane and I don't get any indications of a vacuum leak.Checked the fuel cannister purge valve, it's ok. I smoke tested the engine and not a bit of smoke. I should also say the truck runs great. Idle rpms are right on and is super smooth. The long terms have actually crept up slowly over time from where they first caught my attention at around 10+ and both sides have come up togrther gradually.The short terms stay very low and the long terms don't actually jump around, once they get to that spot they stay there. It's got good fuel pressure sitting in the yard and doesn't lack any power on the road at all so I've kind of ignored any fuel supply issues. It's got about 90,000 miles on it and plugs were replace at about 60000.I will say the fuel injectors on this truck look kind of nasty. I had to replace one a while back and the plastic jacket was cracked from metal underneath the platic jacket rusting and expanding as it coroded.They all look like that.I've wondered if they're all wearing or clogged just enough to increase the pulse intervals and skew the LTFTs. Wellcome any comments.Thanks!

[@MotorAgeMagazine]

What are the STFTs? What are the trim values at idle? At 2500 rpm?

[@jeffreyallen3777]

@@MotorAgeMagazine Short terms seem to bounce back and forth pos to neg 3 points or less either way. Never any big corrections The long terms will start out at 7 or 8 cold start and then slowly build into the 20's over 4 or 5 minutes. At 2500 the long terms will drop pretty quickly to about 7 or 8 positve and stay there until I drop back to idle and this is about the same driving..I should mention I replaced exhaust manifolds on both sides so I checked for leaks but did not find any . I checked the bell end connector of the pipe just in front of the sensors especially and there didn't seem to be any leaks.

[@MotorAgeMagazine]

@@jeffreyallen3777 And this is not setting a DTC? Usually, high trims at idle and more normal at higher rpm indicates a vacuum leak.Did you smoke test it with the engine at operating temperature?

[@jeffreyallen3777]

@@MotorAgeMagazineToday I did a second smoke test after reaching operating temp. I plugged my smoke machine into the brake booster hose because I know where it connects to the intake on the back is a problem area on a lot on triton motors. I couldn't see any smoke and didn't even catch a whiff of it and that burning oil has a strong smell. . I'm going to check the exhaust carefully for leaks again as both headers were replaced a while back but I can't hear anything and I don't see any soot.I hope they haven't warped again. I don't know if that could cause pos trims this high anyway. I have another f150 with a 4.6 and it threw a code a while back on bank 1 and that's exactly what it was. Strangely it coded at trims around 12+. You could see soot on the exhaust bell and it was making quite a bit of noise on hard acceleration. I tightened up the bell and that solved the problem. Anyway, thanks for your thoughts, much appreciated.

[@jeffreyallen3777]

Just to follow up I smoked the crankcase and found a hole in the cast aluminum valve cover on bank 1 that was hidden pretty well by the fuel/ignition wire bundle support stud. It was at least a1/4" hole. That's a lot of unmetered air.

[@isrealperez9069]

Can you help me with this on b1 (stft) - 100 ltft 7.45 b2 stft 0 ltft .78 04 BMW 325xi 2.5l

[@Uomarfr]

I get P2097 post Cat rich then after about 10 more miles I get P0420. My long and short FT do not go over +5%. It is 2015 Chevy volt. Do I have bad Cat or Downstream O2 sensor?

[@MotorAgeMagazine]

Is the downstream sensor an OE part of aftermarket replacement? Have you tested its ability to react to shifts in air:fuel ratio?

[@Uomarfr]

Motor Age the car has only 39k miles. I have not replaced any of O2 sensors. When I bought the car it had a crack between MAP sensors and intake manifold which I replaced it couple moths ago, I did not notice the crack when I bought the car. The car that I replaced: 11-15 Chevrolet Volt OEM Air Intake Tube To Throttle Engine Cover 23452493

[@Uomarfr]

No, I have to tested air:fuel ration. I can buy one and test it. Do I need to test upstream or downstream or both? What is the normal number should look like?

[@MotorAgeMagazine]

The P2097 is tied to the response of the downstream sensor while the P0420 is tied to the comparison of both. I'm going to say replace the rear sensor with an OEM unit and clear the codes/data.

[@Uomarfr]

Motor Age thanks, will update you after it is replaced and passed all the modules test.

[@psions555]

I'm having a hell of a time understanding my Mercedes at the moment it used to run a LTFT trim of 3.9% max. it made small short trim corrections. Now STFTS swing up to +/- 25% either way and LTFT is running about +8% sometimes goes over 10. I was always taught any trimming over +/- 10% is a concern. No fault codes, passes every DAS test, engine runs fine but I can just feel and hear something is off 😭

[@MotorAgeMagazine]

Monitor your fuel trims at hot idle and at 2500 RPM. Post the total of STFT and LTFT here and let's see if we can figure it out.

[@psions555]

@@MotorAgeMagazine I thought I did a reply with a video id uploaded maybe YT took it down? Got in the car this morning and gave it a little rev and there was even a few little pops and bangs from the exhaust which is telling me that there's way too much fuel going through there. Still no error codes or fault codes from a vehicle with a fairly comprehensive and complex diagnostic system. 2500 RPM hot idle I've got short-term fuel trim going into the negatives yet the long-term fuel trim is running into the positives? What gives? I thought negative is rich so ECU will decrease injector pulse and positive is a lean indication so increase injectors? Why would it's short-term strategy not reflect in its long-term strategy? Seriously puzzled

[@MotorAgeMagazine]

@@psions555 Send me your vehicle info - Make, model, year, engine, trans. Email it to [e-mail]

[@robertpaul3675]

I recently changed to premium gas last couple tanks

[@sleepyheadp1926]

I’m having a hard time with P0420 . New cat new plugs both burned up . Guessing too rich . My fuel trims are . SHRTFT1(%) LONGFT1(%) MAP(psi) RPMrpm VSS(mph) 6/15 15.6 -16 22063 1439 77.25

[@sleepyheadp1926]

Shortft1 15.6 Longft1 -1.6

[@TONYDERU]

About your Alcohol / Gas /Water test: When alcohol is absorbed (Dissolved), into the water, we should see the volume DECREASE. Your test procedure is also incorrect. It needs to be performed like this: Individually measure the amount of fluids and then combine them. The combined volume or height will be less than the amounts mathematically added together. Similar example; Mark the level of water in a jar, add a good amount of table salt without stirring or agitating, and mark the level/height; it will be above the water only line made earlier. Then stir the salt and water solution until most or all the salt is dissolved into the water. Next, view the level of the water-salt solution and notice the level has dropped and returned to the first marked water line. Alcohol also dissolves in water like salt does, but ( I'm not certain if alcohol actually dissolves or just mixes with gasoline ) doesn't dissolve in gasoline.

[@matteo3893]

I been looking for a solution for my 2007 civic 1.8 fuel consumption problem. There are no codes stored but have been getting terrible fuel economy. My ltft is -10 to -14 and my stft is almost always in the negatives. Any suggestions?

[@MotorAgeMagazine]

Interesting! The fuel trims indicate that the ECM is actually providing less fuel than normal. How are you judging this as a fuel consumption issue?

[@matteo3893]

Motor Age the car used to get close to 400 miles to a full tank. As of lately I’m lucky to get 260 miles. Did a compression test it came back OK Fuel pressure is in spec

[@MotorAgeMagazine]

@@matteo3893 How is overall performance?

[@matteo3893]

Motor Age when it gets to operating temp it starts to act funky. Especially when jabbing on the gas it sometimes hesitates. Overall good but there is drivability differences from a cold engine to warm. There are two ect sensors one on the rad and one on the stat housing. The upper one seems to be operating correctly from what I can tell using a scan tool but I’m not sure how to test the lower one

[@MotorAgeMagazine]

Start by verifying the basics. The first thing I'd check is injector flow based on what you've shared so far. I suspect you have one or more that are leaking.

[@Boosted3.7_AZ]

What could cause high LTFT’s of 16% at wide open throttle on a supercharged speed density car?

[@MotorAgeMagazine]

3 questions - 1. Any codes set? 2. What is STFT under the same conditions? 3. What kind of car are we talking about?

[@Boosted3.7_AZ]

@@MotorAgeMagazine No codes set at all. Around 3000RPM of going Wide open throttle, LTFT goes 1.12 bank 1 and 1.16 bank 2 for the complete run… STFT goes 1.20 B1 and 1.27 B2 for 2 seconds… then lowers to 0.98 to 1.01 B1 and 0.91 to 0.98 until I let off. It’s a 2017 centrifugal supercharged V6 Mustang.

[@MotorAgeMagazine]

@@Boosted3.7_AZ If your STFTs are normal, your ECM is in fuel control and happy. I wouldn’t worry about it.

[@TheJayscar]

How do you figure out what is causing the computer to think it has high ethanol?

[@MotorAgeMagazine]

Have you tested the fuel's alcohol content?

[@TheJayscar]

Yes and it showed me there wasn’t really anything.

[@MotorAgeMagazine]

@@TheJayscar Have you checked for related TSBs? What make/model/year are you working on?

[@TheJayscar]

Motor Age yeah and everything is normal except the ethanol percentage. And it’s a 2008 impala v6 flex fuel

[@MotorAgeMagazine]

@@TheJayscar Have you tried a KAM reset?

[@rickivictrimusiq]

My Ltft is +14 at idle...when i accelerate it goes down but as soon as i take my foot off the gas pedal it goes back to +14. Mazda demio 2005

[@MotorAgeMagazine]

If there are no codes and STFT is switching nicely over 0 - then don't worry about it. 🙂

[@gmwrenchead]

l have never been first... before! Thanks Pete.

[@Coondawgwoopwoop]

...That's what she said...

[@robertferrara4529]

Another brain teaser, What is the best way to identify a leaking injector and isolate which cyclinder is in? Thank you once again,

[@MotorAgeMagazine]

I think I covered this answer in the response to your question on this video: kzbin.info/www/bejne/apq8c62GaNCIoM0

[@Destructive_Asylum_13]

Hi I need some help. I rebuilt a 2001 vr6 vw motor has 268 cams & titanium valve kit. I keep getting a p1137 code I been chasing for 1 month. I have changed maf,injectors, purge valve, fpr, fuel filter , o2sensor still keeps coming back.

[@MotorAgeMagazine]

What diagnostic tools do you have access to?

[@Destructive_Asylum_13]

@@MotorAgeMagazine torque app scanner. I also have the basic small scanner that shows stft & ltft maf

[@Destructive_Asylum_13]

@@MotorAgeMagazine ok i got some data from the torque app scanner. Maf at normal running is 6.0g/s STFT: 2.3% 02 1×2: 0.4V LTFT: -14.8% VOLTS (AD): 13.9V FUEL FLOW: 38.6CC/MIN FUEL FLOW: 2.4L/HR UNDER LOAD 11.8% MAF: 15.2G/S STFT: -10.9% 02 1×2: 0.6V LTFT: -14.8% VOLTS(AD): 14.0 FUEL FLOW: 92.9CC/MIN FUEL FLOW: 5.6L/HR hope this helps. Thank you for helping me. I just bought another set of injectors & was gonna install today after work. But ill wait till I hear from you. I also for got to add that when I initially start the car it cranks for awhile I gotta step on the gas then it will start.

[@MotorAgeMagazine]

So you had this problem from the start (after your modifications)? I like to watch the STFT in action. Is it swinging over 0 equally, or biased one way or the other. Is LTFT climbing? For now, what size engine is it? And what is the MAF reading at warm idle in grams/second? What are the trims at warm idle? What are the trims at 2500 rpm? Power ok?

[@Destructive_Asylum_13]

@@MotorAgeMagazine yes had this problem from start i believe since I put the rebuilt motor in. Its a 2001 vr6 vw gti 2.8L So at warm idle STFT jumps between -5.5% TO 12.5% Maf is at 4.0g/s 02 1×2 0.1v-0.6v LTFT -17.2% CONSTANTLY FUEL FLOW 25.9CC/MIN AT 2500 REVS MAF 12.2G/S LOAD 9.0% STFT -17.1% 02 1X2 0.1-0.8V LTFT -17.2% FUEL FLO 68.5CC/MIN

[@kristopherhasenbuhler5393]

What would cause my STFT to read Zero???

[@MotorAgeMagazine]

STFT should constantly shift, positive to negative and back, with "0" as the center point. Are you in "closed loop"?

[@kristopherhasenbuhler5393]

@@MotorAgeMagazine no lol i just realized that after i typed this question.

[@mrprevost1]

Bad forward o2 sensor on back 2 was the problem. Runs like scared dog now

[@johnjoseph7382]

lol

[@Tootoojackson]

I have short term bank 1 25.0 Short term bank 2 25.0 they both goes back down to 0.0 then go back up to 25.0 what can this be the long term is good 0.0 my car is running lean p0171

[@MotorAgeMagazine]

If your STFT is off at idle and OK at higher rpm, you are most likely looking for a vacuum leak.

[@Tootoojackson]

@@MotorAgeMagazine I find all the leaks...I was looking at my Scan tool I find the fuel system 1 and fuel system 2 had a open loop fault so that's is telling me my upstream 02 sensors about to go out

[@oSTYNCLSYo]

At idle my LTFT is 21.1% both banks ?

[@MotorAgeMagazine]

Is this a MAF system or Speed Density system? What happens to fuel trim when you bring rpms up to 2500? What are the STFT numbers at idle and 2500 (add the two together for Total trim)?

[@oSTYNCLSYo]

Motor Age its a maf system. When I idle up the LTFT drops but still positive and stft goes negative. While driving the rpms bounce and there's a delayed shift sometimes. I will get rest of the numbers

[@MotorAgeMagazine]

Generally, if total trim is positive at idle and become more normal at higher rpm, the culprit is a vacuum leak - or any source of unmetered air that is getting into the engine. If the trims are normal at idle but trend positive at higher rpms, then fuel delivery is the prime suspect. If trims are positive across the board, it's an input or sensor issue I'd be looking for.

[@oSTYNCLSYo]

Motor Age ok here we go lol. LTFT idle is 21.1% STFT 5.5%...@2500rpm LTFT 4.7% 3.1% STFT 11.1% 14.1%...the STFT does go neg and pos at idle when I tap the throttle. It fluctuates

[@MotorAgeMagazine]

I'd be looking for a vacuum leak. A smoke machine works great for finding one!

[@dennisrisner7900]

What if your fuel trims will not read or stay at 0%

[@MotorAgeMagazine]

Neither should stay at 0%. Short Term should continuously switch from positive to negative over "0" and Long Term will be a reflection of the learned value.

[@dennisrisner7900]

@@MotorAgeMagazine so I might have a short somewhere right

[@MotorAgeMagazine]

@@dennisrisner7900 No, this is not necessarily a problem. What are your fuel trim numbers doing?

[@dennisrisner7900]

@@MotorAgeMagazine nothing . Can't be read at all . Even tried to reset the ecu and nothing .

[@MotorAgeMagazine]

@@dennisrisner7900 Sounds like we need to start at the beginning. What kind of car are we talking about? What is your original problem? And what kind of scan tool are you using?

[@AnythingEverythingGh]

My stft stays +25% in all driving conditions, have changed the 02 sensor still having this. Trouble code thrown is P0171 Help

[@AnythingEverythingGh]

@DIY Garage nissan sentra 2008 2.0l petrol cvt It's a maf sensor

[@MotorAgeMagazine]

@@AnythingEverythingGh OK, next step - what are the total (STFT + LTFT) trims at idle and at 2500 rpm?

[@AnythingEverythingGh]

@@MotorAgeMagazine (25%+9%)=34% And all driving conditions, even at idle

[@MotorAgeMagazine]

@@AnythingEverythingGh Then I would suspect a problem with the MAF sensor.

[@AnythingEverythingGh]

Changed it still having that problem, my next culprit is fuel injectors. Possible to be the problem??

[@hacatan24]

My fuel trim isn't moving sensor 1 does but not sensor 2

[@MotorAgeMagazine]

Need to be a bit more detailed. What kind of vehicle? Year, make, model, engine...? Are you referencing Bank 1 Sensor 1 and Bank 2 Sensor 1 OR Bank 1 Sensor 2? What are the fuel trims themselves reading?

[@stevenmchenry9276]

Hey Pete I'm still totally lost can you make more videos breaking it down even more simpler & more basic & slower for the few people like me.

[@MotorAgeMagazine]

Sure Steven! I'll put it on the calendar.

[@isaiahhicks1314]

I have two shft readings were it just sits at 99.3 and 21%

[@MotorAgeMagazine]

I've never seen STFT at 99.3 on an OBD II vehicle. What are you using to access the data? What is the yr/make/model of the vehicle? What conditions are you looking at (idle, cruise?)

[@isaiahhicks1314]

Using OBD2 machine I think these two readings are off the back 2 sensors

[@isaiahhicks1314]

Yea I have that two on back sensors I assume those sensors are not needed

[@ysnphotos]

👍🏻💐😊

[@chrisczarnik3439]

Excellent presentation for me, just got a scan tool and trying how to use it. Old mechanic from 60 years ago.👍👍👍👍👍from 🇦🇺🦘🦘🦘🦘