The Trainer #65: Fuel Trim Explained - What It Is , How It Works, And What It Can Tell You
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Published 2017-04-28
Most technicians understand the concept of fuel correction and what we mean by “closed loop” operation but just in case…
Fuel control is the concept of maintaining the proper mixture of fuel and air in the cylinder for efficient combustion. Incoming air is measured (directly or indirectly) by the Engine Control Module (ECM) and then the ECM chooses how long to open the injectors in order to add the proper amount of fuel to that air charge. The original injection time is based (in part) on the known flow of fuel through the injector but that original number is not perfect.
So a feedback sensor is used to tell the ECM how it did. These are the conventional oxygen sensor or the wideband oxygen sensor. Based on this feedback, the ECM adjusts the next injection event by increasing or decreasing the injector on time as appropriate. This process is continuous and results in the Short Term Fuel Trim (STFT) numbers you see on your scan tool.
Ideally, the ECM will adjust the on time to cause the STFT to swing from a positive number to a negative number with each adjustment, with an average approaching zero. If the adjustments the ECM has made are insufficient to do that, the ECM “learns” a more permanent correction called Long Term Fuel Trim and adds that factor to its internal calculations. If the total amount of correction (STFT + LTFT) exceeds a programmed threshold, the ECM will record a “system lean” or “system rich” DTC and turn on the Check Engine light.
But do you really understand the diagnostic advantages these Parameter Identifiers (PIDs) have to offer? To the knowing eye, the trims can point to the cause of that lean or rich system DTC. They can also be helpful in determining the cause of a misfire, or even identify a clogged exhaust.
Watch this month’s edition of The Trainer to learn more!
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All Comments (21)
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Look old man, I’ve been a senior master ASE tech/mechanic for over 25 years, after listening to you going on and on about your diagnostic theory I feel the need to say…… thanks. I’m still learning. Just subscribed and I’ll be going through all your previous videos and checking in for new ones as well.
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You run into people that are true Teachers/Trainers in life, AND you Sir are one of them. Thank You
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Thank You 🙏 Your videos are keeping this “Old Timer” relevant in today’s Automotive world, you have no idea how much this helps me, 45 years of wrenching and I’m learning more now than ever.
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why is there only 311 comments ? this channel is LEGENDARY, or is it above the heads of most 'TECHY'S ' ?? keep up the great work, BLESS 👍😍
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Great explanation. Best 20 mins I’ve spent on YouTube in my life.
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This lecturer cannot be more perfect in substance and delivery! Outstanding!
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I just fixed my car with a P0171. Did almost every test and came to the conclusion that my fuel filter was the culprit. It can literally be a million different things. Fuel trims are your best friends. Along with 02 data
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I just had those codes show up on my 2002 Toyota. Ran the MAF procedure from their Service Bulletin and saw that it was out of spec. Replaced and fixed. Apparently it was under reporting the air weight.
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This is a very comprehensive explination of fuel trims and how short term and long term fuel trims relate.
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New sub explained in an incredible easy to comprehend explanation, Thank you. At 70 I can still find well explained info.
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Great video. I learned it slightly different but, basically the same in that the computers response could be compared to an old style tv. The short term is like a fine tuning and the long term is like changing the channel when the fine tuning reaches its limit.
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Absolutely Outstanding videos You are an amazing instructor! Thank You Thank You Thank You !!!
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Been studying your videos past few days thank you for your wisdom and knowledge I appreciate it
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That was one of the most useful 20 minutes of my life! Amazing presentation both in terms of content and delivery style. You have a new subscriber.... (with a new Creader VIII diagnostic tool).... Thank you :-)
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Hey I’m just a parts salesman who uses a scanner daily this video has helped me tremendously understanding dtc’s. In instances were I would’ve sold a spark plug, maf sensor or 02 sensor for a trouble code, now are instances of bad vacuum lines and air leaks that are more apparent seeing and understanding now. Thanks again great video
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This was great at helping me understand and diagnose issues with my fuel trim.
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Very nice presentation from the ground up— helps so much in understanding how it all fits together!
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I can always tell when somebody knows what the subject is he or she is talking about, that is when I begin to understand also. Thanks for being that here, well said and explained.
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Just bought a blue driver scan tool and wanted to learn. then i found you! Awesome channel ! I'm a new sub. Thanks for the wisdom !
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I have tried numerous times to explain this to rookies. I must say that I would not make a very good teacher. That is why I refer them all to either watch your videos or find out more info on their own. Believe me also that you can also teach an old dog new tricks. Thanks again, this is a great video. Keep them coming.
