Simple Fuel Trims Explanation (STFT and LTFT)

[@philipcbarrera1537]

One of the best in explaining what Fuel Trim does and how to isolate a drivability issue.

[@dwilliams321]

Thank you for teaching me in 10 minutes what my professor failed to teach me in half the semester

[@colchilibeck]

I glad you found it helpful!

[@noahcalhoun8719]

Yeah I know same here. Teachers been explaining fuel trims for a week and this taught me more

[@vegasking7022]

You guys didn't learn it because you guys were probably the ones who slept in class or was out flirting with the girls during the lab curriculum.

[@mohammedsiddeg3948]

I have not liked or commented on a video in my life but the explanation you did made me do so ……hats off 👏🏽👏🏽

[@melwebb8485]

Love the simplicity its finally made me get my head around fuel trims and understand how it all works.

[@dieseldan1]

Best explanation of fuel trims I've seen yet! Made it so much easier to understand.

[@warriorsf911]

Probably the best fuel trim explanation I’ve ever seen on youtube. Thanks for sharing.

[@ZealoustheEnder]

I'm a visual learner. Thank you for explaining fuel trims.

[@malcolmmarzo2461]

Excellent teaching on your part. An old adage applies to teaching: The way to learn something is to teach it.

[@explorehunter]

Clearly explained the myth about fuel trims 🌹👌👍.. very good illustration and description.

[@pctuterman1]

Awesome teaching. Love the visual. Really makes understanding easy.

[@eddd6827]

Thank you for taking time and explaining!!! Very easy to understand! Thank you so much! ❤️

[@labradormcgraw]

Great explanation. The pictorial representation works really well. Thank you.

[@jacobouso8285]

Thanks, you finally made me understand after trying to learn this from various sources in vein.

[@dougwatson703]

Best explanation of fuel trims I've viewed, thanks!

[@calholli]

It makes more sense to think of it as an AC signal floating on a biased voltage. The Short term trim is your AC signal alternating, and the Long term trim is a bias that follows the short term action, to account for what it is doing, that way the short term can drop back to zero and continue to alternate, while the long term is holding the offset.. It's so much easier to show it on a graph this way; and I know it's hard to explain in words like this.

[@paulwells4203]

Well done, it reminded me of the Khan Academy videos my kids watch to learn Calculus etc. Concise, no excessive talking or bad jokes you give a clear explanation of a topic that will help people diagnose their car's problem. Thanks again, Subscribed.

[@sollykhan2385]

ohh yes, the 'KHAN' academy, glad were doing something positive 😇

[@StationRussification]

Long Term is the window of time the adaptive memory dictates the fuel trim, Short Term is live data calculating to learn the parameters for the adaptive memory to update the Long Term. The O2 sensor signals lean below .45 volts & rich above .45. The Engine Controller only cares about that threshold & uses that to determine fuel trim. .. in other words the computer calculates the % of time that the O2 sensor is past the threshold of .45 volts & does that each time the threshold is crossed.

[@tonyharris3205]

thankyou so much!! ive been fighting this exact rich issue on my built evo x and im going to tackle diagnosing my regulator tomorrow thanks to this video! literally these exact symptoms youve described for rich conditions

[@BWSRbushcraft]

Very well and simply explained...thank you

[@MaverickSu-35]

At last. A much better video explaining the fundamentals behind the fuel trims. Now..., I have a Focus III 1.0 Ecoboost (2015 Facelift) and it had about 25000km when I bought it (second hand) and everything was perfectly fine..., until my bad habit of trying to test things the hard and costly way, brought me with an issue that I haven't been able to solve for 2 years with all of the diagnoses done by a Ford maintenance facility and some other Ford "know-how" guy. I have poured a "fuel cleaner" in my tank at a very high dose compared to that recommended (some 10-15 times higher dose) thinking: "If I put more, nothing should go wrong, it's suppose to just clean, right...?". Well, guess what, the engine started to run very rought at idle with a lot of misfires and after eventually that gone away over time, the fuel consumption went to almost 20% higher than it was before adding that crap into the fuel. From what I think, the solution within that fuel cleaner has somehow got stuck on both O2 sensors, making them tell a lot of lies to the ECU, which takes the O2 data as true data. Since I've meddled with that cleaner solution, my LTFT remained stuck somewhere at positive 11%. Sometimes it goes to positive 12% and above. That is interpreted by the ECU as a vacuum leak and "thinking" that there's too much oxygen within the mixture, it pumps more fuel for a given air mass. The ECU does it's job correctly, but I'm more than sure that both O2 sensors tell the ECU that too much oxygen is coming on the exhaust, such as..., running leaner than it should..., which is very false. At least this is how I see the problem. I would like to understand from you if indeed the O2 sensors can foul the ECU to believe that there's too much oxygen being taken in the mixture or not and also to know if you think that the nuisance cleaning solution might've damaged the fuel injectors somehow and thus the faulty fuel injectors might be to blame for the interpreted too much oxygen, idk. Looking forward to how might see this issue! Thank you very for your effort Cheers!

[@a.kahnman615]

The best so far

[@trealwilliams1563]

You've hit the nail dead-on, Grrreat Work 💯🖖🏾👍🏽

[@johnmitchell2741]

This video was extremely helpful . Thanks for taking the time to make it😀

[@homeroJoseGarrido]

great video !! way more simple and easy to understand thank you

[@francisdsa6790]

Very good explanation and demonstration appreciate thank you 💐🙏

[@tehmtbz]

Very intuitive explanation.

[@bharada80]

I am from Indonesia, thanks very good....

[@earldaisgomez3959]

Dear sir. Good explanation. Thanks

[@laneheron8677]

Great explanation

[@mptrader1620]

This was an excellent presentation and explanation. Thanks. What if STFT never return to a normal range and LTFT also are high 12.5% during this time. Obviously there is some kind of leak but why would the STFT not return to normal especially under load. The car runs fine otherwise.

[@chadrose]

Thank you for the teachings

[@batughandurkaya4033]

Very helpful thanks a lot !

[@lugnut1009]

Very good explanation! Thank you

[@MarkSmith-js2pu]

Excellent job!

[@bertcochran1770]

Very good information, thanks so much.

[@mauiztic]

Simply excellent

[@raimondtutu3248]

Brilliant explanation!

[@masterofnone7846]

Amazing stuff here!

[@adanarceo4636]

Thanks for sharing this valuable information Colin. This is easy to understand because the way you explained. I really appreciate it brother. I m your newest subscriber. 👍

[@AngelSaysHey]

PERFECT explanation, Thank you

[@bdub3871]

Awesome video. Thank you

[@pedroluizoliveira2417]

Thank you! Bro Great explanation

[@SeanBaker]

The LTFT on my 09 Prius with 251K miles is at -1.6%. This is after I changed to Shell VPower+ gas, and I cleaned my iniectors. Before this, LTFT was at -3.91%.

[@ehasaranga]

thank you very much for this explaination

[@FranciscoPerez-go8ys]

Very good explanation thanks

[@nhlanhlamagagula7467]

Thank you for the review I now have better understanding, it appears we need to check the long term stats more than short term since the short term fluctuates and long term is the reading of the short term over time ..Please let me know at what percentage should one investigate or preventive measures, I believe the MIL lights above 25% either lean or rich condition base on long term fuel trim..

[@colchilibeck]

If you see anything over 10% (+/-) then there is something going wrong.

[@olegkukhlevskiy6006]

@@colchilibeck -/+10 on LTFT right? Not the small one

[@colchilibeck]

@@olegkukhlevskiy6006 correct

[@olegkukhlevskiy6006]

@@colchilibeck tnx for the reply, i have -25 st and -0.16 on lt when I accelerate, im ok right?

[@colchilibeck]

@@olegkukhlevskiy6006 I wouldn’t worry too much about what it is under acceleration. What is your long-term at after you go for a drive? If long-term is still fairly negative after a drive and it idle, you could have a fuel delivery issue.

[@FREEPROGRAMS22]

Great explanation there 👍🏾

[@dannybailes3948]

Very helpful. I may have a medium vacuum leak

[@DinkyDoughnut]

Gr8 Mr.President.!

[@freedom6919741]

Great video !

[@johnjauregui3528]

How does ethanol affect ECM/PCM fuel trim calculations? Does it require the same 14.7:1 ratio calculations or some other ratio? If so does the ECM/PCM sense whether the fuel is ethanol or gasoline and use a different ratio for ethanol? If the ECM/PCM assume gasoline is used and does not adjust for the use of ethanol what is the impact on air/fuel calculations and fuel trim outputs? How does this affect scanner outputs, rich, lean, etc.? Is there a YT video that addresses these questions? Bottomline: How does the ECM/PCM adjust for these two different fuel types with different optimum air/fuel ratios, or does it not, and what are the impacts on scanner readings and diagnosis? Again, is there a YT video that addresses these questions?

[@monkey875]

What causes, fuel weak pump,air vacuum leak ,bad ignition weak spark. In my case was vacuum leak. Thanks I understand fuel trims a little better. 😊

[@ryans413]

So after an hour drive my STFT was around -4 to +6 and LTFT always seems too be at -15 and only gets up too maybe -11 -5 is that normal

[@mohammedhasan118]

بارك الله فيك

[@sajanaabeywardana9725]

Superb explanation

[@badmintonlesson]

I have a questions... My vehicle. LTFT -8.6 steadily..... STFT is moving +- 10 How can i understand about LTFT Negative (-8.6) continuously ??

[@georgemikadze3648]

Well thank you so much for perfect explaination, just on question, So if everithing is good (ideally), both indicators stft and ltft should tend to 0?

[@colchilibeck]

Ideally they will be around 0. STFT will move around a lot while driving which is normal.

[@LuckyLe7]

Pretty clear. Thank you

[@alexanderoronov4392]

Very good explanations, except that you messed up the negative and positive trims. If the displayed fuel trim value is a negative number, it means that the ECU is decreasing the injector pulse width to subtract fuel from the air/fuel mixture to lean out the air/fuel mixture because the input data it is receiving tells it that the mixture is too rich. If the displayed fuel trim value is a positive number, it means that the ECU is increasing the injector pulse width to add fuel to the air/fuel mixture to enrich the air/fuel mixture because the input data it is receiving tells it that the mixture is too lean.

[@jonathanmonger111]

You're the best

[@prospexican]

im still watching, but what was the matematcs to keep shrt/long trim within 10% of each other?

[@mattsmith2739]

Hello, is there a easy way to tell if o2s is bad...Make more videos this one was pretty good

[@Jytimbalero4632]

Nice reasoning!

[@notmenotme614]

What is an acceptable amount for LTFT, to be working within limits? In the past it had fouling of one of the plugs. causing a misfire. When the plug is clean the misfire is fixed. When I connect an OBD2 reader to my car and idle after warmed up (closed loop). The STFT acts as expected, hunting from 0.8%to 2.3% However the LTFT remains stationary at -11.7% 02S1 slight fluctuation from +0.07mA to -0.09mA O2S2 slight fluctuation around 0.7V

[@ryans413]

I get the same but LTFT is like -15 after an hour drive it only gets up to -10 to -5 never goes zero and STFT hovers around 0

[@lupas20]

I feel like the drawn should of been different. I understand the reading of - means rich and + means lean. I would say the red ball should be on the rich side because of the reading I think it may give some confusion. When the problem is indicating a long term rich reading we will be around -15/-30 when our issue is found our short term will then run + because our issue is correcting and rebalancing back to our 14.7.1.

[@atifabusalim7217]

Very nice vedio But i think it better to practicall Please do this in soon future

[@kenya2012asia]

Very useful. I just have a question. My case is LTFT -24%. STFT is almost 0% at idle but If higher rev range over 2500, STFT will be 15-20%. What happens in my car? If LTFT is based on STFT, LTFT will be slowly to 0% or a positive %. Also, the STFT increase at higher rev range is an issue? Or normal?

[@WV591]

I miss cars with carburetors and no computers.

[@ra.dawkins1441]

Well done.

[@andresyt03sg6]

Legit had that issue where ltft would stay at -21 and stfu would be -25 for the longest on my 2018 Honda civic hatchback today I changed the fuel injectors and boom back to normal stft goes up and down like normal - then positive when driving and ltft stay between 2 or 3

[@valentinsofroniev165]

Thank you so much!

[@McLovinMcFlurry]

definitely helped thank you sir

[@pageleftblankintentionally]

So….what’s physically happening when LTFT comes into play? It’s all the same system. There isn’t some magical secret source of LTFT like this explanation purports. Actually what happens is LTFT shows how far the fuel trim has drifted from ideal. The ECM has learned that it must always maintain the LTFT as its new normal due to the imbalance.

[@nadeem5476]

what should be the ideal most readings of both the trims?

[@YNguyen-xk1fp]

thank you for explanation

[@johnjauregui3528]

how does ethanol affect ecm fuel trim calculations?

[@comaradelee]

hello. i have a question. o2 sensor after cat is stable at 700nv to 900mv, is that normal?

[@MM_in_Havasu]

Am attempting to chase down problems in my '14 Chrysler 300C hemi from a P219A(left bank cylinder imbalance/AFR), nothing wrong with engine with compression test 245 psi in all cylinders across the board and sub 10% leakdown, no valvetrain damage, swapped injectors from left to right, fuel pressure tests normal @ 55-60 psi, replaced plugs w/iridiums and hot coils, retorqued intake manifold to 105 in/lbs, and reset PCM to relearn itself. Car runs better than perfect even after setting this code, happens about every 250 to 350 miles on long road trips. All freeze frame data is within normal range. Engine has shorty headers on it with a 91 octane tune and ported throttle body, problem seemed to manifest itself after the headers were installed and took on long trip. Help!

[@thesetruths1404]

Hello. 2003 Mitsubishi 3.8L V-6, 220,000 trouble free miles. Fuel economy is where it should be. LTFT 1&2 are +12.5% at hot idle. But goes to 0% and stays there when revved to 2,500 rpm. STFT 1&2 are 0% at hot idle. When extra fuel is sprayed on identified leak of intake manifold (leaking o-ring, I used smoke to find) the STFT immediately goes to -6 and then returns when the extra fuel is burned off. Could this intake air leak thats affecting both banks cause the truck to go into a limp mode, giving me these codes? P0335 P2135 P0340 Also, why does LTFT go to 0% when under load? Thanks!

[@exeivier7287]

and what if i have a negative STFT and a negative LTFT ??? for example -10 STFT and -10 LTFT??

[@Constitutionallycorrect]

I think a good way to show would be a sliding weight scale at the doctor's office

[@camaroahmed]

i own a 79 caprice classic equipped with Lq4 engine while i tuning by hp tuner i have notice that STFT & LTFT never read no matter what i do still read 0% however i use 2 o2 sensors and hooked up to ECM but i'm not using BCM at all due to engine swap, any assurances to activate these STFT & LTFT

[@SugathanAravindan]

Nice explanation. Is this true for both diesel and petrol cars ?

[@labradormcgraw]

Just petrol. Diesel engines work differently - they don't have fuel trims.

[@nwreop]

I have a question. maybe this was explained and I missed it. So after the LTFT is set (say your LTFT is +18% and STFT +4%) is the total value of increase 22% fuel trim?

[@SeanBaker]

Yup. From what I learned you add them together to get your total fuel trim.

[@baki4king]

I FKNG LOVE YOU MAN!!!

[@larney37]

It is amazing carbs work at all

[@nadeem5476]

if both the trims are negative then is this a sign of a faulty or extra open injector?

[@labradormcgraw]

I'm trying to get my head around fuel trims. Would I be correct that a vacuum leak would cause the engine to run at a higher idle speed than normal? My logic says it would, but I've never heard of a high idle speed being a symptom of a vacuum leak before. Thanks for your help.

[@ryans413]

Yes I vacuum leak would cause high idle it’s air not being counted for by the Mass Airflow and would probably cause the rpms to jump around cause the computer can’t see the air volume as give the right amount of fuel needed. Plus it would effect fuel trims as well

[@labradormcgraw]

@@ryans413 Thanks for your help.

[@ryans413]

@@labradormcgraw No problem

[@patkelley2190]

exhaust manifold leaks will cause a lean condition. Its unmetter air. When cold leak can be big and when hot it can be small or not at all. Cracks in the exhaust manifold, broken bolts, bad Gasket anything that will allow air in before the 02 sensor.

[@jonathanmonger111]

Well understood

[@Xantylon74]

But what if the O2 Sensor reads the mixture but it doesn’t correct it? Example: LTFT doesn’t move, staying by 0,0. STFT are positive as high as +24 at 700rpm but goes down to +3 or even 0 if the AC is on or the engine is running at 900rpm just 200rpm more? Why does the ECM not correct? Wide band O 2 Sensor, new, same problem as before. Normal O2 Sensor after the catalyst is reading 0,750v. when I take my foot off the gas after accelerating it briefly shows 0.0 volts on then goes back to 0.750-0.800 volts What is happening here? I get how LTFT and STFT work, but this makes no sense to me! If someone has an explanation for this please comment!

[@7150285]

Let's say if u have a leak at the exhaust manifold before sensor 1. Will that cause the PCM fuel trim to add less fuel because the oxygen sensor is showing it's running rich? Because the exhaust gas the leaking out before passing thru sensor 1 therefore telling the PCM is no Oxygen is passing thru the O2 sensor.

[@ryans413]

Yes if the exhaust gas don’t pass through upstream O2 sensor and the downstream O2 sensor after the cat sees the same computer will think the cars not getting enough fuel and add more which it doesn’t need this will keep happening until the exhaust leak is fixed.

[@7150285]

@@ryans413 I thought it's the opposite. I thought exhaust gas contains some oxygen. And if it doesn't pass thru the upstream O2 sensor, it will run lean. Not rich.

[@ryans413]

@@7150285 yes that’s right my bad

[@7150285]

@@ryans413 actually. I think it really depends on each individual situations. Each situations differs.

[@7150285]

@@ryans413 All I know is O2 sensors measures the level of oxygens. Too much O2 means not enough fuel and is running lean.

[@JBB1636]

My STFT is 8.7% and my LTFT is -2.3%, what does it mean by this? I mean, why the LTFT is not also positive?

[@AlwaysDiagnoseVehicleFirst]

Colin Chilibeck Good Afternoon and nice to meet you, is there any way you can make more videos continuing from this one to explain more deeper into STFT & LTFT to get a more deeper understanding and how to use it to diagnose any vehicle please? Also good job on the video you did very good :)

[@ashercohen4739]

thank you

[@alexk163]

Hi, my friend, on my BMW I have STFT 18℅ and long term 11.7℅. What do you think is it air leak? Or a MAF sensor? Or O2 sensor?

[@colchilibeck]

First thing I would check for is an air leak.

[@michaellongrie915]

Good example for a novice to get the jist of it...

[@mobilecommunicationsnetwor5268]

What happens during Cold startup? What data is accessed? Could you explain the FT and ST in these conditions?

[@runezoneful]

Thanks alot

[@faisalnazir710]

Thanks alot sir u did well for us... .sir I have camry 2007 2.4.....STFT -11.72 and LTFT - 19.53.....is it ok or what I will do ....I am in bahrain and here machanic is not well ....Thanks sir will wait for u good reply

[@myasinxx43]

I too had similar with LTFT going to 22.x, there was vacuum leak from break booster pushing more air via break-hose to intake manifold.

[@anthonynicholls8010]

✌️✌️