Secret of Engine Problem Diagnosis- Fuel Trims Pt.1

[@keisha5425]

I’m a non-certified female service tech in my 20’s starting out at a small mom ‘n pop shop. Even though I’m around cars all day and could swap almost anything you could point at it’s been difficult for me to mentally conceptualize how everything ties together or come up with the right approaches to problem solving. Not to mention it’s hard for my bosses to find time to explain things to me and half the time I’m havnt been sure what questions to ask. I’ve been coming home after work to watch your videos oldest to newest (I find the poor video quality to be quite endearing btw haha) and I love the way you describe the basic fundamental rules in a structured and logical path. I have DESPERATELY been needing it!!! It’s been making my day job 10x more engaging and meaningful to me! Everything is more interesting now that I’m starting to understand the genius of design behind it all haha! I am eternally grateful to people like you who give out such valuable information to a public forum. It’s been changing my little corner of life in nothing but positive ways. Hope you see this. Thank you Matt :)

[@SchrodingersBox]

So glad to hear that and I hope this helps you navigate some of the challenges you are sure to find being a female in the industry. I actually have a little helper on some Friday- a 20yr old female tech student who is very similar to you- she is trying to learn the diagnostic concepts as well. I hope both of you cross paths some day!!

[@Dingthedingbat]

Three wires • fuel • spark• gravity • physics. Get past fuel and spark. Except physics, gravity and the elements and the computers control over everything. The big thing is three wires. ⚡️ Old Pablo Tucson, everyone is a boxer or mechanic. Not liking cardboard, I choose mechanics. First thing I said, “I hate the electric shits” If… That’s All There Is. Now I’ve Seen Everything 🔫

[@SchrodingersBox]

Was that slam poetry?

[@keisha5425]

@@SchrodingersBox Some kind of cooking recipe my guess. Not sure what he's making but sounds delicious honestly

[@stuckinmygarage6220]

Nicely said!

[@ranjah007]

As a computer engineer who fixes my own vehicle, by far this is the first video I have seen on you tube that lives up to the standard of being intellectually stimulating. Well done and thank you for taking the time to explain the nuances and logic in detail. I am subscribing to your channel. 👌🏽

[@DougOfTroy]

ranjah007 can you make a program that bypasses all this for a. 98 Dakota

[@The2Coolest2]

Fellow comp engr here! :)

[@d4rk5tar97]

me too

[@jonathanlantz6851]

😢😢😢😢😢😢😢😢😢8

[@robertcribley7791]

😅😅😅😅😅😅😅😅😅😅😅😅😅😅😅😊😊😊😊😅

[@robertdonovan8801]

Techs/mechanics, electricians, plumbers, etc struggle in their respective trades for lack of knowlege on how things really work. Then comes someone like Matt who by previous trade was a scientist but has the smarts to be competent at almost anything. We in the auto repair trade are fortunate to have his perspective and would do well to take advantage of what he can teach. I have been in the auto repair field since 1977 and just had greater understanding of Fuel Trims since watching this.

[@mikestewart9407]

He's a badass.

[@1marcelfilms]

Of course anyone with a functioning brain wouldnt work those trades.

[@mikeam3871]

I’ve worked industrial process control related to precision furnace combustion for over 40 years and you’ve explained the fuel system in modern vehicles in a concise and easy to understand manner that surpasses any teaching moment I’ve ever had. Thanks for sharing.

[@juakaliautomotive2439]

This series is the absolute authority on the matter of Fuel Trims/Advanced Driveability Diagnosis BY FAR -- I've come back to this series countless times in the past three years and they have helped me grasp more on the topic than in the previous 8 years of my career as a tech -- Many thanks sir!

[@SchrodingersBox]

thanks my friend. This video is actually used in several tech schools as part of the curriculum in fact. I think it’s the most important video on the whole channel. probably 80% of all my diagnoses start by using fuel trim.

[@blk7gxn]

FINALLY someone who explains THOROUGHLY and so professionally, that makes understanding the intricates easy. THANK YOU for your videos, your a true car enthusiasts asset. Will stay tuned in to learn more!! YOUR AWESOME!

[@gorillagrease2147]

One of the most informative videos in my opinion with a guy who cares enough to share and explain his knowledge on you tube, whether you're familiar with fuel trim or not this was very helpful at least for me .great effort sir . thank you

[@SchrodingersBox]

You're welcome ;-)

[@baburajbaburajalumkundil1249]

Schrodingers Box

[@mameryjazz]

@@johnshaw5001 not sure if you listened but he never said that STFT set a code. Instead, he said STFT is an immediate response to the percentage of fuel that the pcm is adding or retracting... If, in a given case, STFT has reached 20-25%, meaning pcm is adding 20-25% more fuel to keep the engine running somewhat normally; after a while, this condition is set as the new normal thing to do by the pcm to keep the engine running and the LTFT percentage (The fuel trim percentage noticed after "a while") is set to this 20-25%. Therefore, now the pcm takes this increase seriously and set a code for the pro to figure out what has caused this anomaly. Note: if you are a pro, why don't you contradict intelligently with details instead of just denigrating with a sentence in which you claim to be a pro? Where is your youtube video on this subject? Share your knowledge Mr Pro. If what you must say is not more important than silence then BE QUIET HATER!!

[@maxsmith2320]

@@johnshaw5001 what would be a fix for P0430 code.

[@ymisoflyg8612]

@@SchrodingersBox would you be able to help me out...pretty much figured had a vac leak (06 envoy) ..when warm stft is -3/3 and left is 10-12 ranges...when accelerating stft goes up to 5-6 ltft comes down to 7-8... but sometimes the scanner will jump and show stft like 5-6 and ltft will.be 3-4... and when cold and idling the numbers jump around a bit..

[@jefsmack1]

All I can say is one outstanding person to describe fuel trims. For about seven years I have been digging for information on fuel trims kind of hard to understand but this just nails it!!!! Such valuable information to have. I just have to make sure that I can retain it. Got to have this for future thank you brother.

[@daviddowns7552]

he nails his classes. very understandable!!🏁

[@jonhancock7326]

Found your channel today while browsing for info on my 1998 Pontiac Trans Am WS6 with intermittent hard shifts. I am OEM and technician in the wastewater industry. I have grown weary of so-called professionals throwing parts at my vehicles without proper diagnosis. Hence, I do much of the maintenance and repairs on my vehicles myself. I am certainly not at your level of expertise. However, in diagnosing problems with pump and control systems in my field of expertise, I have to rely on good data. Obtaining good data requires critical thinking. I appreciate your passion for understanding mechanical systems and your persistence to make sure a particular problem is properly diagnosed. Thank you for sharing your knowledge. I also enjoyed your video of diagnosing the exhaust restriction for an older lady. I am glad to know there are others like me who share old school values such as not charging for work that was of no benefit or value to the customer.

[@tarnfeather09]

You are a god! Using what I learned from this video I solved a too lean code in one drive cycle, leading to a failing MAF. Replaced the MAF and am now driving with a smile and without a fault code.

[@zebshahab]

the best ever video regarding fuel trims on the internet no one explain the concept like this

[@asuto120]

shahab alam zeb you are right .. its best video ..

[@ziaziaei5614]

What an amazing, clear, comprehensive explanation of STFT and LTFT and their relevance to engine performance as well as trouble shooting process. Thank you so much.

[@DavidUM-ui6es]

Matt: I'm sorry this is so long, but I hope you find it interesting. Your video provided a rational and super-helpful guide for me to follow in trying to diagnose a problem with high fuel trim at idle. (Fuel trims 20%+ at idle, improving at higher RPMs.) Following what you explain in your video, I came up with the hypothesis that I probably had an intake vacuum leak. After much testing though I had to conclude there was no vacuum leak. Testing also showed I didn't have a fuel delivery problem - no problem with fuel pressure, injectors, O2 sensors, exhaust leak, exhaust restriction, leaking cold start air enrichment device. I cleaned the MAF, tested MAF voltage at various RPM/load settings and checked the MAF wiring harness/grounds. All tested normal. The only unusual finding appeared when I compared fuel trim against MAF grams/second output on a scan tool. At steady idle, the MAF grams per second reading was varying by as much as a gram, and the lowest grams per second readings and highest fuel trims numbers seemed to coincide. Eventually I had to consider the hypothesis that the MAF was underreporting idle air flow to the ECU. I found this article very helpful: www.motor.com/article.asp?article_ID=839. The author concludes the article by saying, “If you can't locate a fuel or air delivery problem to explain high or low fuel trim values, don't overlook incorrect MAF...input values.” Cleaning the MAF with CRC MAF cleaner made no difference. But after I replaced the MAF sensor the fuel trims returned to normal. (LTFT Now 0% with STFT +/- 5% at idle). The grams per second reported from the MAF at idle now varies across a more narrow band, and never falls below 3.5 g/s. (With the bad MAF it was falling as low as 2.8 g/s.) Replacing the MAF fixed the problem. I decided that replacing the MAF was a reasonable thing to do, even though my testing hadn't absolutely proven 100% that the MAF was bad. Replacing the part seemed prudent, but seemed to fall short of the standard of empirical/scientific justifiability you have set. I guess you could say that replacing the MAF was itself the empirical test that proved the MAF was bad. But that kind of thinking seems like a slippery slope that could lead to becoming a mindless parts replacer. What other tests could I have done to test the MAF before I replaced it? The voltage tests I found in the Factory Workshop Manual and in Paul Danner's book suggested that the MAF was fine. I spent a lot of time testing and ruling out hypotheses related to vacuum leaks and fuel delivery and I was running out of patience. Even though I thought it was a reasonable thing to do, before I could bring myself to replace the MAF, I had to overcome a psychological hurdle: Namely the sound of your voice in my head saying, “If you replace the MAF, you're an idiot!” As much as I want to be a member of “the 2%” and not “a mindless parts replacer” I guess sometimes practicality demands a lesser standard. I think the theme of “I want to be a member of the 2% and everybody else is an idiot” has great rhetorical appeal and gives your videos an enjoyable dramatic character. You might find this interesting to read. It comes from a summary of the theory of drama elaborated by the great American philosopher and literary critic Kenneth Burke. He says that all successful dramatic narratives include a victimage or scapegoating mechanism: “Victimage is the process of scapegoating. Here, the speaker blames an external source for his ills. According to Burke, there are two different types of scapegoating, universal and factional. In universal scapegoating, the speaker blames everyone for the problem, so the audience associates and even feels sorry for the victim, because it includes themselves. In fractional scapegoating, the speaker blames a specific group or a specific person for their problems. This creates a division within the audience. The victim, whoever it may be, is vilified, or made up to violate the ideals of social order, like normalcy or decency. As a result, people who take action against the villains become heroized because they are confronting evil.” (from wikipedia en.wikipedia.org/wiki/Dramatism) The dramatic narrative you've constructed in your videos is an example of fractional scapegoating: it is the mindless parts replacers who are the villains, and it is us - the tiny minority of scientists - who are heorized because we are confronting their stupidity. This isn't a criticism by the way. Burke's claim is that all dramatic narratives work this way. And your video was invaluable in providing me with a strategy to follow in forming and testing hypotheses. But eventually, in the process of finally fixing the car, I also had to - temporarily I hope - embrace the role of idiot.

[@Zion-jw3gm]

David UM1996 Two years on and still making people smile, great post. ☺

[@JoeSevy]

Thank you for sharing your experience. I'm working on a 97 Ford Ranger 2.3. I had been noticing sort of poor gas mileage for months; then got a check engine light. Scan tool showed a potential problem with the charcoal canister purge valve. Not interested in empirical testing when I can just grab the part off another rig (But, like you said. That's an empirical test too.) Since I was right there with the scan tool, I pulled up my fuel trims and noticed right away that I had long term over 20, though it would drop under load. Back home; I checked for vacuum leaks and came up with squat. I watched this video a year or two ago, and thought I'd watch it again, to see if my memory of fuel trims was faulty. But first I scrolled down through the comments. Your comment about hearing his voice calling you an idiot, or otherwise denigrating you is almost always on my mind when working on a car, and it's saved me some pointless part replacing since I discovered this channel. I just thought it was funny, how we were in just about exactly the same frame of mind. My wife walked in and asked if I fixed my truck; and I told her: "Maybe. At least what I thought was a problem. But my fuel trim's really high; and I don't seem to have any vacuum leaks. Which is about all I can think of that would cause it... but I guess a bad MAF could do it too..." And here's your comment. Same problem as mine. (apparently) I've been suspicious of my MAF for quite a while now, and I'm thinking that's probably where I should be looking next.

[@anthonypapaleo1940]

Joe Sevy I far from being a ace mechanic but would it set a code for the maf ? I’m dealing with a code on a gm p1174 just wondering if it’s bad gas ? I also think the plugs which are iridium but there at 116000 and I know that it’s need to be changed and go from there .. check for leaks .. there all saying you got change all your injectors and get reprogram not so fast .. the car starts on a dime took on the highway with good response to throttle.. any inputs thanks

[@JoeSevy]

@@anthonypapaleo1940 You're really talking to the wrong guy here for that particular code especially! However; I can't see any possible logic in suspecting the MAF in this problem. The MAF detects total air flow to the engine; it doesn't have anything to do with the detection of a rich/lean imbalance between cylinders. The MAF can be, good, bad, or ugly without giving that code. If you're using a non oem air filter especially, I can certainly recommend cleaning the MAF with MAF cleaner, and after that many miles it's probably taken a bit of a beating. But, obviously, your only legitimate reason for changing it at this point is the same one that applies to changing the plugs. As (expensive) maintenance rather than waiting for it to fail. But, I'd be shocked to find that it had anything to do with your situation. If your plugs have 116,000 on them, I'd suppose they're about due for a change, but I certainly wouldn't have much hope for that to solve the problem as there'd most likely be a misfire p0300 code along with with the p1174. Though I can tell you that I've changed plugs with 144,000 miles on them that were obviously in really bad shape without ever throwing a code. So, if you try it and it helps: Yay! Just don't expect much. "Bad gas"? I suppose it's possible, but not too likely from what I can find out. I'm rather skeptical about something getting sucked out of the tank and past the filter being bad enough to mess with the injectors, but only enough to cause this code. Though, it's entirely possible for the filter to be clogged badly enough to interfere with fuel pressure; again. I'd expect another code, p0174 and poor performance. Though your code is associated with injector problems; injector problems are also associated with engine idle and power problems. Since you don't have idle or power problems, as far as you can tell, I don't think you're too likely to get anywhere there either. Still, you're the one who feeds it gas. If YOU think you've been giving it bad gas, start giving it good gas. If it sits for long periods of time, then good gas could have gone bad. But again. The whole engine gets the same gas. You're looking at an imbalance between the cylinders. Go ahead and use gas additives if you want. Some swear by them, some at them. My opinion is they sometimes do some good, and I've not seem them do any harm. Just don't expect it to fix the problem. I'm also rather skeptical, for the same reasons, that one or more of your injectors has a problem, sufficient to cause this code, yet not bad enough to throw another code or cause problems with idle or performance. There's no way I'd even think about putting a set of injectors in it unless that's something I was already considering for some other reason. You can do a simple test yourself on the injectors to look for obvious potential problems. Simply disconnect the electrical connector to each injector and check the resistance of each one. If none of them are greatly different from the others, they're unlikely to behave differently when they receive a signal from the ecu. This may be a lousy way to diagnose injectors, but I've done it successfully before. The problem is that this can only find one possible problem. The injector could well be damaged, perhaps enough to cause your problem, without this test telling you anything. But, it's something you CAN do, and it WILL eliminate one possibility. You can also check the voltage being supplied to the injectors, but I'd expect that to show up as a code as well if it were the problem, and if it were a fault in the wire I'd expect the injector to completely fail, again, throwing a code. I'd much more expect the problem to be one with an intake leak of some sort, which can be very tricky to track down. You'd suppose a leak would automatically be worse at idle than at cruising speed, but that's not always the case. Finding a leak that only presents itself at speed is going to be quite a problem. Still, that's where to start. Change the plugs, because it's time; then start checking for leaks. If you find one; that's something that needs fixed at the very least; and a very likely cause of your problems. Another symptom of a vacuum leak is if the engine races at startup. If your engine, when started, revs to a significantly higher rpm when started than it used to, or than other similar engines do; you've likely got a vacuum leak. Since all the likely causes other than vacuum leak or general engine deterioration usually cause idle or performance issues, and you're not reporting any idle or performance issues, and there're no other codes, I really doubt that you're going to get anywhere with the injectors or fuel system. The most likely culprit is going to be a manifold leak of some sort. If you have some kind of scanner that can show you fuel trims in real time, I'd expect long term fuel trim to be somewhat high if you have a small vacuum leak, anything over 10 and I'd almost guarantee there's a leak. If you don't have a scanner of any sort I'd recommend getting a cheap BAFX bluetooth scanner; buying the Torque Pro app, and using that until you can get something better. I can't make any recommendation for what that better scanner would be because I can't make up my mind what scanner I'd get even if I could get anything I want. But, even if you can't tell the engine's getting more fuel when you use propane, starter fluid, or whatever to find your leak; your ecm can, and it will change the fuel trim to compensate. That's why you need to be able to monitor the fuel trim as you're looking for your leak. So, what help have I been? Not much. The problem with so much of this stuff is that it gives us problems that are only problems because we're informed about them. If not for this code, you literally wouldn't have a problem to fix. Or at least that's the way it sounds to me. Or, maybe it just gives us the opportunity to fix a small problem before it gets to be more serious. Hopefully I haven't just added my ignorance to yours. Hope you fix your problem!

[@anthonypapaleo1940]

Joe Sevy Thanks joe my idol seems a little choppy but not bad but not racing .. also this code came up after my wife fill up at this one station.. so I’m taking the car to have the plugs changed , maf clean , throttle body clean ... I really don’t know what was done this car I just bought three months ago and also do my own maintenance but do the cold here in Chicago and no garage ... But like I said this only code I get no miss firing or any other code .. I really don’t like throwing parts at and till I know for sure . I will have the shop run a scan and see what’s up with .. thanks for any suggestions...

[@philip1529]

You are fantastic been fighting my 03 dakota for months kept getting 440 codes checked my vacuum lines found three damaged lines replaced them in a few days got misfire code checked COPs changed spark plugs did not get another 440 code but kept getting random timed misfire on #7 changed my fuel injectors (sorry I was one of the 98%) mechaniced all my life but at 65 all my experience was with old school cars frustrated I searched the web after several articles going every which direction finally came upon your video pulled my battery terminal off for ten minutes then reattached drove for 30 minutes ran great MPG increased and no miss. WHAT A MIRACLE you didnt save my life you saved my life style LOL . Have subscribed to your great channel THANK YOU SO MUCH

[@D.Ghost92]

This is probably the most comprehensive explanation of engine issues. I hope you are well!

[@SchrodingersBox]

thank you i am doing well indeed!

[@D.Ghost92]

@@SchrodingersBox hey I'm so sorry for this question, I have a Stft of - 25% and a Ltft of +7.4. Does that sound like a leaky injector??

[@SchrodingersBox]

It could be. Among 10000000 other possibilities.

[@D.Ghost92]

@@SchrodingersBox 😂 true, I should drive the car and see if it corrects itself

[@SchrodingersBox]

lol. Personally I would curve the fuel trim against load as shown in the video to narrow down to the system likely causing the adjustment on the STfT.

[@emrsngs]

Well, Ive either been to idiots or scam artists. I've had the O2 sensor replaced, intake manifold, MAF sensor (2x) once I replaced myself and the second time the mechanic did it and charged me $400 for a 10 minute job, replaced the fuel pump, and on and on. Thousands of dollars later I still have the problem. Thank you for this video. You explain things very well for those of us that are novice auto DIYers. I think I'll be my own mechanic from now on. :)

[@aaronl4131]

Thank you so much for making this video!!!! After 2 days of head scratching, I was finally able to pinpoint my problem to a clogged cat once understanding how to interpret fuel trim data and correlate to O2 readings. Your videos have greatly enhanced my diagnostic abilities. Many many thanks!

[@siskiyouwoodsman4279]

Hands down the best explanation I’ve ever seen on 02 sensor voltage telling short and long term trims what they need to do for the perfect 14.7-1 AFR. Can’t wait to to watch part two. Very useful information, thank you!

[@SchrodingersBox]

Thanks 👍

[@BlackOWLfly]

Thanks to you I was able to self diagnose the MAP sensor problem - it not only saved me money from useless repairs but it prevented me from selling the car, which, by the way, was still having warranty...the dealer was telling that the car was working perfectly fine with a fuel consumption of 16l/100 km in town, from a 1.4l gasoline engine (when new, it never jumped over 11.8l - and that was only in traffic jams). I bought a diagnose tool, recorded some live data from almost 60 min. Of continuous driving in various conditions and found out that the pressure reading was wrong...changed the sensor and now my car works extremely fine. The funny part is that the dealer never admitted that anything was faulty and only a mechanic was interested in knowing how did I determine an issue when no error code was present. Thanks a thousand times for the help!! If you ever write a book on these car topics, put me on the list and I'll be among the first to buy it!

[@RickyPisano]

I learned more in this video than 3 months of binge watching other channels. I worked on cars every day for years back in the day. The old V-8s, Slant/Straight 6's, 4 bangers. Carburetors, Distributors. Coil, etc. Then....I stopped for 25 years. Didn't open a hood for over 2 decades. Recently I purchased a 2005 Ford 500 Limited with the 3.0L DOHC Duratec engine. The car was like new and owned by an older couple. They actually dropped the price from $2000 to $1750 because they found out I was a Veteran. Unbelievable!! I wasn't even going to haggle with them. They had all documentation on maintenance, the car had 5 brand new tires including a spare and not a spec of rust. Impeccably maintained. The best used car I ever bought. When I opened the hood I didn't even know what I was looking at. It was UNBELIEVABLE. And I used to swap out engines in one day by myself. All kinds of jobs. Worked on A LOT of cars in the garage on my house. Every make and model. It was INCREDIBLE to me how much engines changed. I started watching videos on this engine and everything started to fall into place. It was still an engine. Just COMPLETELY CONTROLLED by a CPU. But that was all I really learned other than a few good tips. This channel has brought me back to the point of having enough confidence to do work myself on these new engines. I can't thank you enough. No other channel I watched EXPLAINED why I would be doing a job. Just the how. To me.... the WHY is more important. If you are an old mechanic lost in the technological advances on CPU controlled engines you're in the right place. Trust me. This guy is THE BEST. Hands down.

[@SchrodingersBox]

I was wondering when you would get around to this video lol. In my opinion this one is the most important on the whole channel.

[@johnwebb710]

Great video!! Thank you so much. I had a check engine light on my 2000 Silverado. Both PO171 and PO174. Hooked my scan tool and checked the short and long term fuel trim. Found the long term steady above 20 percent. Took a closer look and found a vacuum hose had come off. Plugged the hose in and no more light. You saved me a couple hundred and I really appreciate it. Thanks so much!!

[@aldohernandez8259]

John Webb great

[@patrickbarry6100]

'If you get this code and you replace the O2 sensor, you're an idiot' This guy is my Spirit Animal.

[@CHIBA280CRV]

This lesson is opening a whole new world . Thank you

[@markmetz1]

beyond helpful! after MONTHS of suggestions from others checking wire connections, solenoid tests, checking wires for continuity and voltage and so much more to then checking my ECU for proper voltage as well as the MAF, (while going bald i might add lol) i followed this mans directions with my OBDII and it showed EXACTLY what he said, i found i had a vacuum line off creating the problem all along. now long term fuel trims are slowly going down as itll take some driving to finally adjust back to proper settings but the code is gone! by the way this was on a Mazda RX8 rotary and still correct info.. meanwhile others were telling me to purchase a new ECU.... thank you so very much for your efforts to help your fellow man!

[@GhostTrader37]

December 2024 - thank you for the informative video, the nuances explained. The logic. It all goes together nicely. Yes ive watched this twice. But wow now im gaining traction on fixing my diesel engine. I appreciate you man. Love from Scotland.

[@inadaaze9968]

Can't thank you enough for making this and your other videos. I learned to wrench on cars back when they had oil bath air cleaners so understanding the PCM has been a real challenge for me. Your videos break down how they work in an understandable way so I can troubleshoot to compotent level and not just guess what part to swap out. Keep up the great work, you are an awsome teacher. Thank You.

[@Tarnished9200]

Any aspiring mechanic that wants to make this an occupational life style choice. Watch this video until these consecpts are understood.

[@MrJGarcia13106]

This video definitely helped me understand the difference between lean and rich . The content was very informative and clearly outlines possible scenarios that might trigger these symptoms.

[@labradormcgraw2409]

This video is worth a fortune! By that statement, I mean it could potentially save anyone who takes the time to fully understand it, thousands of dollars in wasted repair bills. If any would-be mechanic only ever watched one KZbin video on how modern engines work - let it be this one. I think it's the scientific background that gives this guy the edge over the others. Unbeatable knowledge.

[@SchrodingersBox]

Thanks man. Of all my videos this one is both my favorite and by far the most importance. about 80% of performance diagnostics stem from understanding the concepts in this video. At very minimum, bifurcating the causes of symptoms as resulting from a rich or lean condition in itself eliminates 50% of variables. It is extremely powerful.

[@robinsonlantigua5908]

Amigo this tutorial was so cleared and helpful that with the little English I know I understood everything that you explained. Before watching this I was going to spend around $100 on a new MAF sensor, by using what I learned here I found the real problem and kept my original Toyota sensor. Thank you so much. So to show my tech teacher appreciation, I'm spending those $100 dollars buying your shirts. To all of you out there, knowledge is priceless. If you learned something, give something.

[@SchrodingersBox]

Glad it helped!!

[@SugarMakesMePOP]

This is sooo helpful! I'm learning fuel trims for the first time in school and it can be kind of confusing but you do a great job of explaining it clearly. You'd be awesome to have as an instructor. Thanks so much!

[@laschell59]

This video is more the Brain at heart. I love intelligence and the graphics of discussions, 100%

[@geraldinflorida]

This was EXTREMELY helpful for me, one that has not even unwrapped my new scanner. I took tons of notes. I have an Explorer that runs really rough and it's burning tons of fuel. I am about to watch Part 2 and then I will begin my interrogation in the morning. My only complaint is that you did not provide your name. Your art skills are just fine. Thank you again sir.

[@johnbarto4611]

just call him the Fuel trim Whisperer...................

[@nissanzamyadtech.4856]

After 1year of field experience now i am able to fully undrestand what you're saying. Fuel trims are very really importent .This is a really great video and thanks for sharing it.

[@SchrodingersBox]

Glad it helped

[@skbetts1947]

THANKS for presenting this otherwise difficult to understand information in such a GREAT easy to understand format. Your expertise is evident, AND your teaching skills are SUPERB. I am a very old school mechanic that cut my teeth WAY before computer controlled and sensor inputs. Good old needle valve controlled carburetors and point based distributors were then the norm. You have FINALLY helped this old geezer (72 years young) to understand the scan tool readings and how to use them to diagnose common problems in TODAYS engines. My Honda Civic (2004) has almost 225,000 miles at present and finally set it's first P1172, P0134, and P2271 codes. I ran right out and bought an O2 sensor.... but problem persisted….Duh! Now I am equipped to properly approach the problem. THANKS ARE NOT ENOUGH but will have to do. Thumbs up my friend.

[@HealthvsWealth4me]

Wow, I found this very helpful and wanted to give this guy a long THANK YOU for his energy and passion, but I see that other people have done this in great detail ...so I gave a quick thumbs up.

[@czardrum]

You are a monster. Such clear and nice teaching. Greetings from Brazil

[@SchrodingersBox]

+Carlos César Machado Lol a monster! I like it!!

[@troopbd]

+Schrodingers Box your an idiot, i have been an ASE master tech and GM world class tech for a little more than 20 years and diag cars with fuel trim issues on a daily basis and no way does a MAF fuel trim get worse with higher rpms. I will show you my credentials, lets see yours. Quit spouting off shit that you have no clue about, well the rest you were right about but you are honestly totally wrong about MAF fuel trims getting worse with higher rpms.

[@SchrodingersBox]

troopbd I dont have to call you an idiot- you're doing it yourself,. Show me ONE case study where a vacuum leak got WORSE fuel trim under load and better under idle OR even one case study where a dirty MAF showed normal fuel trims under load but higher at idle and I'll publically aplogize. If you don't do that I'll just delete you from this channel- or better yet, delete your comments and leave the ones that just make you look like an idiot. GO! Time is wasting!

[@troopbd]

+Schrodingers Box wait till I get off work, at the dealership, and I will pdf numerous ones and send them to you.

[@SchrodingersBox]

troopbd CAN'T WAIT!!! I'll cover it in my next video with your comments!!

[@kiwiexotics1982]

2019 and still good advice from the day it was made. Im new to Fuel trim but understand it all so far AWESOME. Onto part 2...

[@danielrevo5456]

Just fixed my daughters 2002 toyota rav4 2.0 codes were p0171 p0174. Watched your fuel trim videos and determined it was a lack of fuel. Long terms were high. Changed fuel filter and replaced fuel injectors. Bam fuel trims went 0.0 long and short fixed 👍 runs like new ! Cleared codes no check engine light. My obd2 reader shows green good to go. Thank you, you are the man !

[@JohannesC-c9k]

Great explanation. I had the engine management light on intermittantly, car ran great. Plugged in scan tool long term fuel trim was around 8% short term 16%. I tried this a few times watching the live data when someone else was driving. STFT fell to 0% LTFT has not moved yet but I know it takes longer too respond. If I had a small vacuum leak I would have expected the LTFT to fall. I will keep an eye on it. Had code PO1071 for a while but found another code PO170. I erased the codes, engine management light out and so far staying out. Keeping watching your videos, great educational value thank you Matt. I am a great believer in proper diagnosis, your videos give me a much better understanding of what is happening. Incidently I read if the P170 code returns the fault is most likely the car's electronic control module. We shall see.

[@kinglangren]

Thanks for the information, I’m finally digesting this after a few years.

[@algismalickas632]

I am feeling like I am at a univeristy. So much information and its very accurate and usefull.

[@alanresendizdelacruz267]

Great video and I’m definitely going to continue the series! I even wrote down notes to refer back to while I got my scanner tool out. Thanks!

[@SchrodingersBox]

That is awesome!

[@ccrdrevil2]

I'm a Toyota Master Technician and I am attempting L1 ase to obtain Master diagnostic Technician for the first time. Your videos really help me study for the test. Much appreciated!

[@ccrdrevil2]

Just wanted to say thanks again. I just took my L1 And I passed! That makes me the only Master Diagnostic Technician ever in the history of my dealership! Much appreciated! I will be sure to reccomend your videos to anyone at my dealership who is attempting ASEs.

[@donniejohnson7499]

Autozone. You killed it. They sell parts. No troubleshooting. Your videos are awesome. Your amazing great troubleshooting skills.

[@pedrodelasancha516]

Wow, I'm glad I came across your videos and I can't even thank you enough for sharing the knowledge about all this. I'm knew to working on cars and one of the things I like to do is understanding why things fail as opposed to just replacing things. This is the best video I've seen in regards Fuel Trim, something I knew very little about but the way you explain it in your video makes it very simple to understand. Thanks a lot my friend.

[@housecallmobilemechanic6107]

I can't get enough of all this information... thank you so much for all the great help

[@SavNout01]

I just 'Feel the love' Everytime KZbin brings me to this channel from the host... Ty

[@What1see]

I just wanted to tell you I’m a truck driver. I spent about 4 to 6 hours a day watching this. I did a couple semesters in school for mechanics, but it didn’t get me anywhere. I learned how to throw parts at cards that was it. I’m ready to start my own business. I’m gonna study this the next six months. I will be a paid subscriber soon. I just need to get a stinking PayPal account.. I appreciate this content so much.

[@SchrodingersBox]

Thanks for letting me know. Good luck in your studies!!

[@labradormcgraw]

Simply the best on KZbin. You'll not find a better tutorial on fuel trims... anywhere.

[@SchrodingersBox]

Glad you think so!

[@mfslyphantom8811]

Hey man, I really want to thank you for your time and your talent explaining this to another mechanic, it’s not over simplified yet you’ve put everything into a much clearer perspective and probably made me a much faster worker. Thank you very much for that, to me there was always a little confusion in how to utilize STFT and LTFT in a proper diagnosis but now i see how and why it’s important.

[@stevenmcadams5918]

You make this so easy to understand ..Much appreciated.

[@tintinkkre]

Amazing video. Thank you so much! I never leave comments on videos, but in your case you totally deserve it. Keep up the good job!

[@davedem92pony]

Went to Lincoln tech. Had a great teacher on drivability but was an super A hole. That was 4 yrs ago. Your videos helped me fix my lean / acceleration problem. 🙏🏻

[@dougoakes9705]

Just wanted to drop a Thanks. After watching your Fuel Trim videos a few times, I was able to diagnose and repair the P0171 status on my 1996 F-150 XLT. Applying the same logic in your training videos, I was able to diagnose it as a faulty MAF sensor. Quick easy test to confirm. Quick easy replacement as well.

[@SchrodingersBox]

Nice work! Yes knowing this stuff is one of the best techniques for diagnosing issues!! it really works.

[@stephenmanion3418]

I did learn and understand a little bit more today!!!!….

[@rogerpatrick3642]

I love your video good info 😄 keep it up I keep looking

[@BasketballAve]

Awesome video keep them coming very well explained and informative.

[@SchrodingersBox]

Ball Out til you Fall out! Much more to come... much more!!

[@justinmiller3114]

I wanted to leave a comment saying "thank you" for making this video available on youtube. Recently experienced p0171 on my dodge magnum and your information helped my pinpoint my issue with ease. Thank you again from Texas.

[@SchrodingersBox]

My pleasure! Hope it helped with your issue.

[@manuelgarza7017]

Thankful i came across your channel, very informative and easy to follow. Was able to diagnose and fix a P0171 on a 2014 Nissan Pathfinder using the LTFT/STFT readings. Have to admit i had to fight my instincts/desire to gravitate to the sensors (MAF/02) as the source of the problem but i trusted the method and more importantly the "data". The data showed higher fuel trims at idle and improved (lowered) at higher RPMs. Started hunting for a vacuum leak and low and behold a bad crankcase breather hose (in the worst spot in the engine compartment) was the culprit. $7 bucks later and the fuel trims are headed in the right direction (just need to let it get to the new baseline and should be good). Thanks again!

[@SchrodingersBox]

Well done! The data will tell you problem if you let it. As you said, most people will just jump to a conclusion rather than interpret a conclusion.

[@jasonfromm5637]

Did that cause any driveabilty issues for u?

[@manuelgarza7017]

Was a while ago now so do not exactly recall all the details but I do not recall any drivability issues. Think it was just the check engine light and maybe some reduced gas mileage that sent me hunting for the problem.

[@carmelbank]

“Brilliant” Thank you.

[@wallyg.2409]

LMAO !!! Matt : "If we have these conditions and you put an O2 sensor in it...you're an idiot"

[@SchrodingersBox]

Hahaha, maybe sounded harsh but it is absolutely 100% true!!!

[@wallyg.2409]

Not harsh at all, my friend. Thanks for all you do.

[@larryhernandez5062]

You forgot the comma before the word and .

[@iggychan007]

I feel like my IQ doubled from one to two. Thanks for the vid :D

[@SchrodingersBox]

Hahahahaha. I know how yhou feel! Mine doubled from 1 to 3!!! Wait.....

[@jvgilyard1126]

Man you are a GOOD teacher! I’ve been trying to understand this for like 3 years.

[@simonstininato5851]

I was a parts replacer even with all the gadgets in my workshop. The sarcasm did pinch but you’re right & I’ll take that squarely on the chin for the wake up call it was meant to be taken as. I will be an advanced diagnostic DIY guy from this point forward & never look back thanks to your videos. I’ll be signing up to your paid education videos.

[@SchrodingersBox]

We were all there once- congrats on being one of the few to recognzie the opportunity to elevate- most people are either to proud or ignorant to do that!

[@bobbysilvers7908]

That's good to learn about fuel trims and you do a great job teaching and I'm personally grateful , I just purchased a 96 pathfinder and noticed the engine had been changed at some point but runs great but bank 1&2 fuel trims was very high so I did all the normal thing's , smoke test and pretty well most all the things in your videos to try to diagnose the problem but the trims were good diving but +25 on idle and it was giving me a fit, so I did every test on that motor I could think of , I'm 62 years old and I've built a lot of engines in my day from pro drag to dirt track and I know how to flow a motor , I can turn the exhaust from rich to lean but I prefer a dove Gray , But back to my fuel trims / the problem was the TPS voltage to high and spark advance to high , some body had the distributor out at some time and messed with that throttle TPS sensor, I lowered the TPS VOLTAGE and had to pull the distributor out and reset it because I couldn't retard the park advance ,, so I lowered the TPS voltage and the trims started coming down and I lowered the advance on the distributor and the trims are normal , and thanks I enjoy your videos

[@curtisalan4402]

You sir are a steely eyed auto man who speaks our language!! Thank you for your time!

[@abzbrk490]

Thank you for the fuel trim explanation I have gone though all the tutorials on KZbin and your explained it perfectly just as the computer reading 14.7.1 ratio. As for the people who dint agree with your tutorial the saying goes We Agree To Disagree. your work is appreciated.

[@jairzinhohinds8285]

Finally someone who explains this process simply and work common sense examples. I can’t thank you enough 🙏🏽🙏🏽

[@SchrodingersBox]

You're very welcome!

[@theskyisfalling_9012]

As a Master Technician with years of training under my belt.... This guy is right up there with all my instructors and fellow "master" techs... He's a "paper tech" that can actually fix vehicles in real LIFE!

[@SchrodingersBox]

thanks, that’s a very nice compliment!

[@rochestertommy9995]

SUPER VALUABLE VIDEO! Watched this video 4 times this past weekend, got to work this am and helped get two cars diagnosed. Using fuel trim live data to understand lean codes. Narrowed down options lead to smoke testing found vacuum leaks on each. Thank you.

[@perryjones8997]

wow....im only 6 minutes into this and I am blown away...great informative stuff in an easy to understand message...off to work now but cant wait to get back with a hot cup of coffee and absorb the rest of this video! I am officially a groupie! Serious, thank you so much for this wonderful presentation!

[@SchrodingersBox]

Thanks! The rest is well worth watching!

[@michaeldanzow7573]

Thank you for sharing your hard work for us to use and understand. Completed a LR4 swap in my TJ, and had it tuned 6 months ago. Everything was perfect. Ran into some issues last week, and I have a bluetooth scan tool. Checked my fuel trims, Long term was +10-12 at operating temp, O2 was at .0-.6V. Followed your guidelines, got fuel pressure kit, it was at 50psi, and when revved 45psi. Too low for these engines. This has a return line with a vacuum operated pressure regulator. Pulled the vacuum off the regulator, pressure went to 55psi, and heled when revved. Put cap on both vacum lines, and went for a drive. The LTFT wen down to near 0 in a couple of minutes, and short term went negative, indicating it wanted less fuel. O2 voltage was going up to .7 and .8 volts, which means it likes the 55psi much better than the 45-50psi. Went around the block, and runs much better, holds the fuel trims at near 0, and o2 voltages are from 0 to .9. Without this, I still would have been lost, throwing money at parts.

[@williamdo9071]

These videos Helped me pinpoint the problem to a dirty MAF, not ECU, not carbon deposits in combustion chambers. Saved $100's.

[@williamdo9071]

Problem: pre-ignition under load. Auto shop #1 dx, carbon deposits; #2, poor quality fuel; #3, ECU malfunctions in setting spark advance. Cardone sells remanufactured ECU, but on their website suggests to evaluate LTFT b4 changing parts. The fuel mgmt is a closed loop feedback control system with multiple inputs from sensors. Thx a million.

[@leslie-tolman]

I've never been so pissed off and so relieved at the same time in my freaking life!!!!! 🤬🤬🤬🤬😂😂😂😂😅😅😅 Well, I now know exactly what I did wrong!! It worked awesome at first, or so I thought!!! 😅😅 You just helped me out soooooooo much, THANK YOU, THANK YOU!!! ~08 scion tc 2azfe ~Clogged cat #1... low funds... yeah, I gutted it. LOVED the sudden change in engine response!! Didn't realize what was happening tho... couple months later: leaking fuel injector into oil... soooo many gaskets and o-rings leaking!! I've drained the oil, new plugs, injectors & valve cover gasket. I've gotta drop the pan & replace the gasket, oil pump reseal, crank seal, was already gonna do my clutch anyway so rear main as well, oil filter housing... good hell. I got learned the VERY painfully expensive and time consuming way!! Valve seals... Probably have burnt valves... hopefully my freaking cylinder head & block isn't warped or worse... LESSON LEARNT. DO. NOT. GUT. CATALYTIC CONVERTER. Should have done a hell of a lot more research, replaced it (with the now much cheaper than this bullshit) header and O2 adapter, had diagnostics ran throughout process and adjusted/tuned accordingly and enjoyed the rewards properly... OMFG. I BROKE MY CAR THINKING I WAS TEMPORARILY PATCHING THE ISSUE!!! HAHAHA send help. Lmao 🤣 THANK YOU for this video but I'm gonna go laugh cry for a bit and then watch part 2.

[@aig0039]

THIS IS GOLD!! Thank You for sharing this useful informations. My car runs bad when cold and I taught that mabe its the O2 sensor. But I found out that it doesnt work when cold. Next I will check MAP, throttle body and fuel pressure regulator. I changed the ECT sensor so tomorrow morning I will see if I figure it out.

[@michaeldunagan7838]

Love the Lecture! I bought an Action scan tool off a KZbinr's video showing jts fearutres. The tool saved me from lifting off difficults 4.3L Vortec manifolds. I saw the fuel trims were to specification to include the sign wave of the 0² sensor. I then looked to the electrical part of the engine and found the distributor cap was arching out. Scan tool paid for itself by a factor of three.

[@compal55]

I was told to change O2 but I listened to this guy cause of this explanation and saved $$$$ it was my throttle body not letting air into the engine

[@anthonydemarco1745]

I totally understand it !!! I learned. I’m an Idiot And reclaimed myself to fix my vehicle by watching this video which I should of watched this before I was an idiot Thank you for the excellent fantastic video

[@michaelserrano7097]

He's right!. I started learning about car repair about 11 months ago. I knew almost nothing about cars, now im repairing them and saving a lot of money thanks to this men right there.

[@rintunt]

I’ve watched a few other professional videos on this topic and yours is the first to explain why the trim cycles rich and lean, which was a mystery to me. Excellent video, thanks for making.

[@SchrodingersBox]

Awesome, thank you!

[@dominicdo]

I agree fuel trims are great diagnostic tool. By now i'm sure any automotive technician working on a modern day vehicle will know what fuel trims are. You have to understand this and many other things in order to diagnose modern day vehicles. This is just a everyday common knowledge but i'm sure there are still people out there that are learning and don't know, so thank you for your video.

[@abderrafikbelal4990]

Every time i had a diag problem i return to your videos to good indestand the function, really the best explanation every time, MANY THANK's

[@SchrodingersBox]

Thank you too- it’s good to know I have a comprehensive resource people can refer to for specific issues!

[@A_Litre_of_Farva]

Incredibly good. Viewers, show this man some love and thumbs-up other comments, it helps the algorithm immensely for him!

[@dirkZigander]

Thank you for your thorough, well edited, succinct, clear, easily absorbed explanation of the inter- relationships Fuel Trims, O2 Sensor values and how to use the captured data sets to yield intelligent diagnosis of related engine issues. Your examples of different issues: vacuum leaks vs MAF vs Fuel Pump issues with the related LTFT, STFT, and O2 sensor values are extremely beneficial to understanding what is being measured and how to use the info . Adding the diagnostic considerations of in-line cylinder vs "V" cylinder arrangements was a great help. I can finally use the Freeze Frame and graphing functions of my Innova scann tool to help arrive at a factual based , correct diagnosis of engine issues in my trucks. Thank you... Tom Jones: New Orleans, La

[@elbertmarks5799]

Tom, Which Innova scan tool do you use? And does it work on a 1993 OBD1 vehicle?

[@chriswilkinson4382]

Thank you so much I’ve been trying to learn fuel trims for a long time and tonight I come across this 2 part series and watched them and I am blown away at how easy you have made it I got all 3 parts of the questions right which felt very satisfying and would like to say a massive thank you to you for teaching me. Do you have a bye me a coffee or similar has I would like to show my deep gratitude for the help you have given me. You are a super mechanic and have outstanding ability in teaching also.

[@SchrodingersBox]

Thanks so much!! Very kind of you. But also you put the time and effort and attention in, so you also get credit for your learning this now!!

[@ironcross420]

Thank you for the education. Your videos are the ones that put into a language I could understand. I was just a wrench monkey, now broadening my Diagnostic skills. I grew up working on non-PCM/ECU, CPU controlled vehicles. Now it's my turn teaching my Father a thing or two. He's Mr. ASE Master / GM / Chevy Master Tech. OBD II was after his time. Most OBD I stuff he did was using a paper clip, counting CEL blinks. lol I'm in the process of being hired as a Auto Tech with the US Navy. I'm not ASE cert, but trained. My new job I won't be doing 'major' jobs. My first auto tech job, I have to provide my own tools and I will be using my fathers tools he used when he was working as a tech. All Snap-On from 62-89. Crazy he tells me how much he paid for them vs retail today... Paid $75 for a 3/8 Flex Bent Ratchet and now it is $175. I'm probably going to get the New ASE cert, the Light Repair and Maintenance cert. So people know I'm not stupid / I know the basics which all it is. Plus I want the patch on my uniform :D

[@humanexe]

Best! And I mean BEST and most informative video I’ve watched on my DYI auto endeavors. I was clapping and taking notes diligently. Thanks.

[@SchrodingersBox]

Wow, thanks!

[@SuperMyckie]

Wow!!!! This is Gold!!.. I'm car tech enthusiast, never seen an information this Rich!!!!! Thanks alot Man, now i am falling in love more with Car Technology

[@SchrodingersBox]

Glad to hear!! Yes fuel trims is only a starting point. There is so much more to learn but this is where I still start almost every diagnostic session.

[@998RC]

Watched this by accident when I woke up after falling asleep watching other stuff. This was awesome! Now I can't wait for my next check engine light so I can read the data and actually understand it!

[@princeclive1913]

Your are a good teacher after many months now I can really understand your explanation I am a mechanic in Malaysia thank you sir

[@SchrodingersBox]

You should send me your email address and I will sign you up for my paid channel for free

[@thepsychonaut3007]

Even though I fell off the stupid-tree and hit all the branches on my way down, you have explained it in such an overwhelmingly simplified manner, that even I could grasp the concept of fuel trim .Thank you .

[@SchrodingersBox]

Hahahaha. Trust me I am sure you passed me up on that fall. It took me months to learn this myself!

[@thepsychonaut3007]

@@SchrodingersBox I have been trying to make sense of this for over a year. I have been to every forum in the known universe, but your video connected all the dots to allow me to finally see the whole picture. Live long and prosper.

[@rwrobs777]

The BEST video on fuel trims hands down !!

[@rwrobs777]

Now Danner is the "Jedi Master" but I think you explain it better in Laymans' terms !!

[@graemer66]

Taken on a 175km trip now all looks good (touch wood) STFT around -1.6 Banks 1 and 2 LTFT around -7 Banks 1 and 2 and motor seems to be running smoother. Next I will check Fuel consumption Thanks for your help. Over here in Australia most of the mechanics I have spoken to about this problem just tell me to just reset the codes, this has made my head hurt for many years, my fingers are crossed. Graeme in OZ

[@Hobblecreekkid]

You should have a million subscribers. You're giving a solid education for free. Love all your videos I've seen. Thanks for taking the time to teach me.

[@SchrodingersBox]

So nice of you

[@hinojosa552]

Your the only one all the professionals that have explained the fuel trims a lot better , I'll will an excellent mark

[@SchrodingersBox]

Thank you however I should point out I am a DIYer, not a professional.

[@CA-pv5ie]

Just stumbled on to your page and am 100% going to digest every single one of your videos - YOU ROCK Mr Schrodinger! Amazing teacher!!! Needless to say you have a new and very loyal subscriber!!

[@SchrodingersBox]

Welcome aboard!

[@farmuhqas]

Maybe I got to your video 6 years late but what an excellent personality and a fantastic way of expressing what seems to be a very complicated system. Hats off to you. By the way I love the sarcasm pointing to the experts out there tgat know zilch. ✌

[@rmp608]

This is THE BEST video that someone can watch about this particular subject

[@SchrodingersBox]

Thank you!! I agree lol!

[@salmanmalik3252]

I never found deep knowledge and concepts on KZbin like Your Videos. Your Amazing Artistic abilities 🤣🤣 are very helpful for 2% guys like me. God bless you always. Please Continue.

[@SchrodingersBox]

My pleasure 😊