Short Term and Long Term Fuel Trims Explained

[@markflowers1953]

I've been a dealership technician for over 20 years and have been working for myself for the past 10 years now and I have heard many people who thought they understood long and short fuel trim but after listening to their explanation I realized they didn't have a clue. Without a doubt you have simplified fuel trim and explained it in a way any decent tech would understand. Most technicians are lost if they don't have a DTC to point them in the right direction when diagnosing drivability concerns. If you have a good understanding of how fuel trim works why and what will make the values change you can diagnose most drivability problems quickly. Thanks for a very informative video worth watching and please keep more like this coming.

[@bigred5393]

Why does he have to be so good. Save the rest he's the best 👍 keep them coming have to say you qualify to satisfy. Thank you sooo good!

[@blue03r6]

Well if you're a mechanic for 30 years you would know if the customer is having drivability problems and its related to fuel/air there WILL be a DTC. Because above or below 10% the MIL will be on because you will start having driveability issues. If the MIL isnt on start looking for mechanical problems

[@kingofring3399]

​@@blue03r6I have that issue , but there is no DTC engine check thou

[@timmer9lives]

Mark....You literally took the words out of my mouth. I've turned wrenches since 1981. Had my own shop for 15 years, got hurt...went to work at GMC dealership for a few years . Kind of retired now and stopped working at a dealership in 2011. All of that said, this guy is not only sharp, but his explanation is spot on and he does a wonderful job of explaining everything. I was thinking...."Man I wish I had someone like him working for me 25 years ago." To the OP, thanks for posting. A very good refresher course for me.

[@fireboy5043]

U the Man Mark u Just Made me a Long Term Short Term Fuel Trim Professional 💯 😂😂😂

[@eso1666]

I’m NOT a technician and after watching this video I can teach others what is the Air fuel ratio and Short/ Long term fuel trim 👍👍. Thank you man!!

[@bigrr64]

this is one of the best explanation of short and long fuel trim I've seen on KZbin. great job explaining it.

[@jrharryman32]

Yeah I agree. Very good explanation.

[@kiddbrother3]

yes, sir!

[@ΓιώργοςΠαπαδόπουλος-κ5χ]

ScanerDanner check him out

[@hacatan24]

He should of explain when it not working at all

[@affanemoustafa2025]

Great explication👏

[@DJMANDARKofficial]

I rarely leave comments. But the quality of this video blew me away. So informative and well put together. I'm walking away feeling confident I can repair my misfire.

[@anserecks671]

I'm a life long D.I.Y. guy and I watch a ton of these videos for different views and perspectives and you my friend are worth the subscribe.. keep up the good work and keep the videos coming..

[@leonardschuhmacher3230]

I have been working on my own vehicles now for almost 40 years now. in that time I have come to know how several of the emission control components work, at least enough to make necessary repairs. I am one of those people that needs to know the "big picture", once I understand where I am trying to go, it becomes much easier to find the path to get there. in the first few minutes of your video as soon as you explained the mandated air/fuel mixture ratio , for the first time in my life I began to see the big picture of emission control. And by the time you finished explaining the role of the O2 sensor , I felt like I finally knew what was going on, and how all these components play together to acheave this one simple little goal, 14.7/1 (the golden ratio of combustion) feel free to use that term if you like, LOL. this was the best video I have ever seen on KZbin. Your explanation was simple but thorough, your graphics illustrated what you were explaining very well. And the progression, and order of your thoughts were logical and methodical . thank you very much for expanding my knowledge, Leonard Oh, by the way do you think the cigar idea has any merritt?

[@bigred5393]

You have to give credit to his clarity I raise my glass of wine and thanks to the man 🇺🇸

[@paulwells4203]

The best explanation on the internet of long and short term fuel trim. Other channels give more detail and end up confusing me. You cut to the basics and explain them in a way that is easily understood. Thanks a lot!

[@hollidaydarwin]

My car had a +15% long term fuel trim. I cleaned the throttle body and 1 of 2 MAP sensor and the air filter. It is now running at 0 to -0.7%. Good video.

[@ahmetsarpilhan]

This is the perfect explanation of a fuel trims i have ever seen in my whole life. Perfect job MASTER ! Thanks!

[@راجيرحمةربه-ي1ك]

I spent a whole week moving between clips and books in order to understand the fuel trim and instead of understanding the complexity of the matter more, I thank you very much for the clear and simple explanation

[@ahmedsalah7474]

JUST BRILLIANT! I have watched a lot of auto channels and this one just beats them all by light years!

[@BeADad2447]

I just got 2 years of automotive tech school in a few minutes! Thank you! You think you know about cars then you watch something like this!

[@IanLandesman]

EXCELLENT video! I've been DIY repairing my car for about three years now, but fuel trims have been a mystery to me. Even after reading several articles, I just didn't get it. A few years back I dyno'd my car to check performance, and a lack of performance led me to monitor my AFR and found a lean condition. I should have looked at fuel trims, but didn't understand them. I changed the fuel filter on my 100K mile car as a stab in the dark. My AFR returned to expected and re-dyno'd the car with expected whp results. After watching your video, I'm going to check my fuel trims soon and I'll have confidence next time I run into a lean/rich problem. Thanks!

[@VAHOSS]

By far the best fuel trim/,O2 sensor video on KZbin. Some people are just really good at teaching and explaining

[@kimokahikolekalihi]

Your explanations are some of the best out there. Thorough, simple to understand, humorous and no BS. Thank you! I learned a lot from this video.

[@heatmizer-jim6762]

Thank you! I work at a parts store, and one of our regulars called last night and was asking about fuel trims, and I told him I would look into it for him. Your vid was the first to pop up and WOW! A GREAT help to me. I think I should be able to help him now. :)

[@SurferSandman]

It was super valuable to see what the O2 sensor readings should be doing! Helped me determine my O2 sensor is bad.

[@markt9438]

You are DA Man ! All the issues i have searched and You diagnose was the only one that clearly made sense ! I have the same condition on my 2011 335i and LTF is running at 15% while my O2 's are doing good so Intake leak for sure ,I suspect Inlet at the inter cooler ,the cheep plastic and one bolt that holds in the rubber ring ,Needs upgrading ! Thank YOU okay update apparently my long fuel trim spikes when I apply my brakes and come to a stop otherwise it runs around 3% and everything else looks great so I had a M3 with a grommet brake booster issue looks like this is notorious happy hunting all this is a place where most people don't realize

[@r42221]

I wish I had college professors that could explain things as well as this guy. Great video.

[@benkanobe7500]

I know this is an old one, but it is still a great one. I am a long time subscriber, life long auto mechanic (retired), and 63 years old. I learn something from you almost every one of your YT I watch. You are a very smart and intelligent young man. Thanks for all you do here on YT. And yes, how do we make our own Smoke Machine. "South Main Garage (auto repair)" is a lot like you, but for us regular folks working at home, we don't have all the cool tools he has. He has a smoke machine (first I learned of one). As a guy working in his garage and driveway, how do I make my own smoke machine??

[@wyattoneable]

I love the white board of learning. Your good at explaining the systems and process of troubleshooting.

[@tonyscalifornia]

Finally you got me on this one. I got a new scanner that I can get me a bunch of info and watching over 10 video explaining about fuel trim. You should be teaching at school sir. Thank you a lotttt

[@greg3663]

AWESOME VIDEO, you taught me exactly what I've been needing to know and understood it the first time around. Glad you added the example numbers

[@georgecolbert3247]

Best video I've seen on the subject. Simply explained without getting off-track. Very helpful.

[@joedso9710]

Great video! Learning alot. I believe your right. the exhaust restriction will cause more of a rich condition. Since unburn gases are restricted by a clog cat unburn fuel will re enter the cylinder unbalancing the mixture. An open egr or clogged egr can cause a lean condition.

[@AVW_Dr.Dr.]

I'm a professor and you, sir, have a natural talent for teaching.

[@bigred5393]

Thank you for sharing your comment he makes the world a better place.

[@BAMAJiPS]

Dude - FANTASTIC INFORMATION! Concise and really shows the why behind the numbers. Great explanation! And yes the Essex engine in general had issues with internal intake seals and the isolator bolts. I think you nailed the diagnosis

[@sb10101]

this is by far one of the best st/lt fuel trim videos on yt. thought i had a grouding issue, but after scratching my head for a minute i looked at the data. damn, saved me $$$!

[@hypetekauto]

the way you explain things makes it so easy to learn, thank you for this.

[@whatisdreup2]

Out of all the videos I clicked on for a simple and straight forward explanation this was the one 👌🏼 ty sir!

[@fieldsofomagh]

Well executed video with a good mix of practical and theoretical.Looking forward to the super duper smoke machine !!

[@joey243win]

getting back on the tools after a 4 year hiatus these videos are awesome for rebooting the brain!!

[@ronabbott2053]

concise and helpful; able to convey the concepts of fuel/air management and trouble shooting efficiently

[@Daniel-uf1xx]

Till this day I still come back to your videos on lean conditions. Sometimes I think to much on diagnosing and I have to take a step back and go back to the basics

[@Buckmaster299]

So glad I found this. This will help me to understand the data logging that I have started to do on my truck. Thank you for taking the time to explain how Long and Short term fuel trims work and how they are supposed to look.

[@8209eduardo]

Thank you for this video, it was really helpful and self explanatory, I was able to determine what was going with my old civic, turned out my map sensor wasn't working right and Idle control valve kept open for longer and my idle was dropping below normal. My long term fuel was at 12.5 % while myshort term was at 28.5% i replaced injectors just to be on the safe side and everything its normal now , long term is at 2.3% short term is at -3.2+% thanks for your videos keep up the good work 😀

[@davemaster001]

Thank you for the excellent explanation. Fuel trims are much clearer now

[@MrPavePaws]

Never had a mechanic explain it so clearly and in depth. I feel like a woman who just learned how to check her oil and I'm all giddy with myself

[@crank3646]

Actually on higher rpm the ratio gets a lot richer than 14.7:1 , especially with turbo engines . This is needed due to the higher heat in the combustion chamber . Fuel is like intercooler , aqueous intercooler . This helps to prevent melting the pistons . For a example , Mitsubishi Evolution have around ~11.5:1 ratio , and the tuned ones can have even 9 or 10:1 , so the mapper can be on the safety side :D P.S I'm about to make my own smoke machine , but going to wait you to steal some ideas from you . Great video as usual !

[@myRatchets]

Hey that's good info, my smoke machine will be very basic and crude! you can get started on yours :)

[@b20339]

This is the BEST explanation of this whole concept I've ever seen, THANK YOU! I've been fighting a P2177 code on my 2006 Mazda 3 for a while and I've never fully understood what those trim values mean and now I do thank you thank you thank you!

[@b20339]

If my logic is correct and both my short-term and long-term trims remain high constantly I most likely have a fuel restriction of some sort because if it were an air leak the STFT should zero out long term?

[@b20339]

My numbers LTFT: 20.3, STFT: 13.3

[@robman38]

This is the exact information that I needed and have been looking for. Thank you so much for sharing.

[@keivanabdollahpour8737]

So professional and to the point, with proof more professional. Thanks. I got a Mazda CX7 with the same mystery, but causing a misfire on cylinder 2 and 3. I think with exhaust leak it's gonna be solved

[@carmenk4018]

Wonderfully informative video. Clear. To the point with no bullshit and honest disclosure. We need more like this.

[@mjl6425]

This is very easy to understand. My turbo car is showing -17 long term fuel trim. I’ve been trying to diagnose a power loss issue and thanks to this video I have a direction to look now. My issue seems to be the exact opposite of this. I’m having a rich condition because air is being lost somewhere and not getting to the engine. So to compensate the EC you is pulling 17% fuel which explains the bogging issue i been having.

[@mjl6425]

Do you have a turbo car? if so you might have a Boost leak.

[@mjl6425]

The short term will fluctuate constantly in real time. You may want to keep an eye on the long term more. I fixed my Boost leak’s and put the bpv back to recirculate. My ltft now is -3 give or take.

[@kiddbrother3]

this is the best explanation of short & long term fuel trim thank you, sir! keep up great work! keep it simple!

[@andrewkinzie9564]

This is spot on exactly what is going on with my Chevy S10. Thank you so much for making this easy to understand. 🤙

[@Case_]

One thing to note when it comes to fuel trims and vacuum leaks that *very rarely* gets noted - you'll most likely only see high positive low rpm fuel trims (lean condition) with a vacuum leak if your engine is MAF based. If you happen to have a MAP based engine (as in engine that doesn't have MAF sensor, also known as speed density engine), your fuel trims will likely be perfectly normal. That's because with a MAP sensor, you more or less can't have unmetered air in the engine - so even if the air is coming into the engine via a vacuum leak, it will be metered by the MAP sensor just fine and therefore will *not* cause a lean conditon (well, it still might under certain conditions, but it's much less likely). But your engine performance will still suffer due to the vacuum leak. Knowing this might save you a lot of headache trying to find a vacuum leak that actually isn't there if your car has a MAP based engine. Trust me, I know ;)

[@JoseCruz-kp7ru]

So how do you find these leaks - I used to spray starting fluid with the engine running in a attempt to locate leaks.

[@shifty2755]

@@JoseCruz-kp7ru Smoke test.

[@david-tracy]

My car has a MAF sensor & a MAP sensor. I just searched them both in google for my 2009 bmw 335i. Seems like what you're saying isn't thorough.

[@shifty2755]

@@david-tracy A maf sensor is air quantity ant temp. A map sensor is the vacuum or pressure in your intake manifold

[@RC-no3uw]

@@JoseCruz-kp7ru you can fin the leaks by relying on your 02 sensor readings if you do have a vacuum leak, the oxygen being higher after the catalyst would show on the 02 sensor readings

[@leonardsolis9876]

I studied for a couple id uears all about fuel trims, but yesterday i was at a loss because i had firgitten iust about everttviny, so i had to watch your video, now im starting to remember. Thank you!

[@shapeshifter3660]

Thanks a lot. i will check my car's fuel trims tomorrow.

[@fasteddie0318]

Wow great explanation, UTI’s instructors couldn’t explain such an important issue! Instant subscriber

[@quicklooksentertainment3217]

Aloha R&W, I recently found your videos and find them very easy and comprehensive to follow. You have good explanations with "why" which all comes together to make sense. Good video shooting too. Keep up the excellent work! Looking forward to seeing more from you. Thanks for teaching us.

[@JasonBrady-e8k]

I love how thorough your explanations are!

[@alienpowergt]

I have question, could placement of an o2 sensor effect fuel trim readings of the engine, for example of the factory manifold had the o2 sensor at the top, and the aftermarket moves the o2 sensor down underneath the vehicle

[@paquesepas6696]

I have a similar question. My Honda elements previous owner had a shop install an aftermarket cat and I noticed they changed the placement of the upstream sensor a lot closer to the cat itself than stock. It's almost on the cat and the downstream is at the normal position in the middle of the cat. If the upstream is very close to the cat basically at the beginning of the cat. How will that affect my fuel trims because I'm running lean like negatives 15 on both short and long term but I have zero codes just a little rough idle occasionally only on idle.

[@wmichaels362]

Good information. Another cause of low O2 readings can be a misfire. I know this is not your symptom, however. Low O2 readings (100mV) indicate excessive oxygen in the exhaust stream. This can happen with a vacuum leak or a misfire. With a misfire there is unburned fuel (rich smell) and the unburned O2 for the misfiring cylinder(s) in the exhaust stream passing by the O2 sensor. The O2 sensor can only measure the oxygen, so it 'sees' a lean condition. The ECM reads the low O2 signal and increases fuel trim (STFT) to respond, eventually the long term fuel trim (LTFT) will increase until a DTC fails.

[@GhaziSarhan]

Dude you're amazing, all your videos are helpful, thanks for the infos.

[@SubOptimal_]

man you explained this way better than my instructors. thank you

[@gertis7420]

This is great info. Thanks

[@myRatchets]

np, glad you liked it. cheers

[@KennethArnoldarnoldservice]

Thanks so very much for explaining the short and long fuel trims! I have never been able to understand this until now! You are an excellent teacher!😀

[@MrBow54]

get a cheap cigar disconnect the vacuum hose on the brake booster blow smoke in and see where the leak is and that's how you make a smoke much

[@aaronreihl2581]

doesn't work with cheap cigar. only cigarett

[@myRatchets]

I think that's what Scotty Kilmer does.. no wonder he is out of breath all the time.

[@StockholmDashcamOfficial]

Scotty Kilmer... What a joke to the KZbin DIY community.

[@Jayson90210]

Did this same thing to find a crack in a small vacuum line

[@TheMrmmkkpro]

Great video , well explained for a old school mechanic , helps me understand what's going on . My S10 2.2l is around 14.6+ lean ,I just put an alternator on maybe I knocked a vacuum line loose , thanks great job👍👍🏁

[@jokjoksson4369]

Yeah, please make a video showing how to make a DIY smoke machine!

[@myRatchets]

Sure, will be coming up in the near future.

[@Blazer02LS]

paint can or something better? my first one was a paint can unit. Then I got fancy and turned a smoke chamber out of stainless and built a home version of a smoke pro.

[@answerace]

Thank you for your awesome explanation. Im diagnosing a p0174 on my 04 F150 4.6 that has 5k miles on the new engine. I bought a scan tool with live data to see if I could find the issue. I've checked for vacuum leaks with spray and used a vac guage and am getting steady 20 psi vaccum off the manifold and the brake booster. I've cleaned the maf and CEL keeps coming back. Will be reading live data once it comes on again.

[@whytewabbit]

Great explanation on short/long term fuel trim via example!

[@jasonrogers9169]

This was extremely helpful. You should be an auto technology teacher!

[@DopeSyk]

YOUR VIDEOS ARE THE BEST!! My problem isn’t exactly like this but I watched anyways because you’re seriously great at explaining stuff. I’m getting codes 0171, 0174 & 0301 and this mechanic says I need to change spark plugs and replace all these pcv and breather hoses and wants to charge me $900.. but I feel like he’s being shady because I oiterallly changed EVERY spark plug the day I took it to him LOL. I’ll keep watching your videos hoping to find the right one for me :) thanks man

[@EATSLEEPFARM]

Older video but subscribed because of it. Really easy to follow and straight to the point. Great video

[@monkeywrangler5200]

Fantastic video. Clear and concise. Gonna send customers to it so they better understand what I’m doing and why.👍

[@Karolilalu]

Great video and superb explanation. I have the exact same readings on my LS3 engine. Suspecting an inside vacuum leak within or around the plastic intake. Again, thanks a lot.

[@Blair62]

That was a great video. Thanks. I learned a lot. The problem is most likely the isolator bolts that hold the intake plenum to the intake manifold. The rubber gets old and they leak. Ford has a TSB out about it.

[@NC-bs6yi]

You're a great teacher my man

[@adimih2120]

I knew how it works from school, but could not remember (and could not easily find online) the 'normal' values...:) I'm glad you gave us the values. Thanks I just subscribed

[@thedudeaaronfyi]

Your just so good at teaching! Thanks for all your hard work!

[@GeraldReeves]

Very good introduction to ST and LT fuel trim parameters - very helpful & recommended !

[@Lockguard]

Great explanation of how it works! This helped me determine a vacuum leak on my car with a persistent misfire.. Now i just need to find it!

[@deseremere]

Thank you, Sir. Very helpful. My old Honda with over 200K miles is showing 4.7% long term trim and I didn't have an idea what the heck it is.

[@EdwinHenryBlachford]

well explained. Conclusion: if observing bad fuel trim replace everything on the engine.. then find accidentally it was a vacuum hose at the back of the manifold, which you can't see without ultrasound.. yas gotta luv puters

[@None-ij7zz]

This guys right. Some intakes are split in half and they have gaskets on the inside called plenum gasket. They look very Similar to the intake gaskets. When you take it apart look for bolts if you flip it upside down, if not the intake is a 2 piece but glued together at the factory non serviceable and have never seen one fail when it’s factory glued.

[@blairj1409]

You are very clear with your automotive suggestions, I appreciate this a lot! I have a 2004 GMC Canyon, automatic transmission, 3.5 litre 5 cyl. The LTFT is +12 to 13 %. I decided to use my new scanner tool to show me live data while I was driving my truck, to see if I could spot any strange or concerning anomalies. Nothing showed up if I was just idling in the truck. I did find something strange while I was out driving on the road test. I set the scan tool to show me only 4 statistics: STFT, LTFT, O2 sensor #1, and O2 sensor #2. So on the drive, the readings of the 4 statistics would fluctuate up and down as a person would expect. Then suddenly while driving at about 90 km/hr, and 1750 RPM according to my dashboard readings, the scan tool all of a sudden showed that the engine was locked at readings of STFT 0.0%, LTFT 1.6%, Sensor #1 at 0.005 Volts, and Sensor #2 at 0.40 Volts. It was my understanding that the O2 sensor # 1 would always fluctuate up and down from zero to 1 volt while driving, if working properly? The scan tool showed these frozen/stuck engine readings while driving at 90 km/hr, and did the same thing at 110km/hr. The fuel trims and the O2 sensors were frozen in the readings, for about 5 seconds to 10 seconds, without changing. Sometimes the voltage of the O2 sensor #1 would very minimally change to 0.015 v, and O2 sensor #2 to 0.050 Volt. I have been experiencing a lack of power and hesitation, at these exact speeds, in the weeks prior, so the scanner findings make sense, I guess. What do you think about this? Any of your experience and knowledge would be appreciated for sure! I’m guessing if it is possible that the lock up mode of the automatic transmission has something to do with it? To me it does not seem logical that while driving and the transmission enters into lock up, that it would ever cause an O2 sensor reading to freeze and the Short Term Fuel Trim to freeze accordingly? When I am at idle, the O2 sensor #1 seems to work properly…it fluctuates from say .850 and all the way down to .015 as a rough explanation. Is there anything that would cause both O2 sensors to die at same time? Oh, I forgot to mention the odometer is at 418, 500 Kms. Well, thank you for all of your informative videos!!!

[@billybobbarker3175]

Don’t see any other replies. If you haven’t got it fixed by now … the diagnosis and repair for your described issue is a failing O2 sensor. It is dropping out when it gets hot. Replace the O2 sensor and your truck will run like a rocket. Hope that helps!

[@blairj1409]

@@billybobbarker3175 Thanks for your advice! The O2 sensor failing once it gets hot makes perfect sense in this situation. Now I've learned something new. Appreciate it!

[@scottjamison1602]

Great explanation and very well explained. I thought I understood the fuel trims, but your explanation.gave me some new ideas.

[@dc5723]

Awesome explanation of ST and LT fuel trims. Thanks very much!

[@MrConspark]

Great explanation, helped me diagnose a few tickly problem child cars! Thanks cheers from Australia

[@tejasgawade]

lots of good information. i will try this on my car. i am facing issue like rpm drop and vibration at idle, struggles when clutch is engaged in first gear. cleaned maf and egr but no success. I think LTFT will help me to get to problem. once again great video.

[@TheJhit16]

One of the best videos I have seen about fuel trims, awesome video man. Looking forward to future ones 👍🏼

[@billmacdonald1533]

Found the problem. It was the injectors. Replaced injectors, spark plugs and injector electrical connectors and the engine runs fine with no codes. Thanks for your informative video.

[@carlnikolov]

This guy knows how to explain things like a boss

[@nathanielmontgomerysr.1227]

Very good, you explained this situation like a pro! Thanks for the insight, it really is VERY useful information for the DIY's in the world.

[@justincole3874]

Best description for fuel trim, watching the video a few times helped me pass engine performance ASE 👌

[@chadrose]

Great video bro, very knowledgeable and you explain very thoroughly. Much appreciated brother

[@chungaleta1234]

Great vid. It is not how much you know, but how good you can explain what you know. You Sir.. Are great at both!

[@user-mr3vw6xg7o]

Thanks for better understanding of O2 sensors 👍🏼🙏

[@SanchoSanto]

I've got a -10% long term trim with a P0420 error code. From what I gathered here the negative trim may be due to the inefficient cat. I'll see if the trim changes once I replace the cat. Thanks for the great explanation

[@ugaugauga488]

Excellent. Really clear explanations and reasoning!

[@JT-iw2cw]

Just by color coding the air/fuel and rich/lean conditions, you have cleared up so much confusion.

[@loscarinte8002]

Outstanding video, to the point and easy to understand.

[@EricKotter]

Great video! Very well explained. I now have increased knowledge to diagnose my car issues.

[@Victor-vp7kn]

Thank you very much for sharing your knowledge. Great explanation with numbers and everything.

[@stevehicks6145]

Dude you’re the freaking man…. What a great teacher.

[@jakehanson6963]

Best explanation of fuel trims I've seen. Thanks man!

[@tylergreen2113]

This is amazing, perfectly and clearly explained. I’m hoping this can help me narrow down my rough start condition on my 07 cobalt!

[@maxsmith2320]

hi, this is the best tutorial on fuel trims and O2 sensor. Thank you