Fuel trims and wandering idle - 986 Forum - The Community for Porsche Boxster & Cayman Owners

I played with my ODBC dongle the other day and noticed my long term fuel trims are at approx -11% for both banks. I also noticed my idle bounces around. It will be at ~900rpm, drop down 600-700rpm and seem rough, then slowly rebound to around 900. Rinse and repeat. If my understanding of the fuel trims are correct, my car thinks I'm getting too much fuel and is therefore running it lean at times. Would that explain the wandering idle? If it does, then what should I be hunting for in regards for the reason of the lean fuel trim as my understanding is it should be much closer to 0. Please be gentle, this is my first time with fuel trims lol! TIA Jason

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To just look at just one data point doesn't tell you much. you need a lot more information to begin any diagnosis. Your -11% fuel trim can be caused from something as simple as aging sensors. or high mileage or? or? or? Long term fuel trims are learned fuel trims. they are learned from short term fuel trims over a period of time. They are affected by things like sensor age, mileage, driving style, even elevation. What you need to know is what short term fuel trims are doing under the same conditions as the long term fuel trims. Also what are your O2 signals doing? Whats the MAF signal doing? throttle position sensor signal??? What RPM? what engine load??? You need to know the specific conditions that generates any given data. If you want to learn about fuel trims, then you have to start with O2 sensor design, function, and there relationship to the DME/ECU. Go to youtube and look up ScannerDanner and his vids on O2 sensor function and fuel trims. I don't think you will find much better info then he provides. your hunting RPM could be due to a dirty MAF sensor. "-But that is just a guess without more info-". You could try cleaning it and see if it helps. Get some spray MAF cleaner. Don't use anything else. Don't touch any part of the sensor except the plastic housing. Don't touch the little wire if it is the hot wire type, or don't touch the little plastic looking film if it is the hot film type. Spray the sensor with the cleaner let it dry well, then replace it and see if it helps. If nothing changes after cleaning the MAF start looking for vacuum leaks.

Thanks! I'll check on the youtube videos. I know it's something I need to understand better. Will clean the MAF sensor ASAP. O2 sensor signals seem to be fine. I'll take a drive in the next week and have my son capture the data at different loads and see where that takes us. Thanks for the response!

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Keep us posted let me know the results of cleaning the MAF. watch Scanner Danners video on how he uses fuel trim waveform and O2 Signal waveform to detect vacuum leaks. Dirty MAF or vacuum leak is where I would look first as to the cause of your hunting idle. Have you had a check engine light come on??

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No CEL. Will definitely update.

Trying to understand more about fuel trims... then again, in my car the idle is spot on, so hopefully highjacking the thread causes no hassle here... This image here is from FlatSix / Jake Raby video and those figures look to be very much uniform: This image below is from my car (2001 986 with 2.7) and the figures are way off compared to above FlatSix values: Are my car's injectors shot or something else there messed up? The car runs nice and has not issues for loss of power not anything else. What can cause such different values that whet Jake Raby's figures show? All help is most appreciated

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First you are comparing dissimilar data. The Raby info is for Adaptation Range 2 (FRA-FRAU) for banks 1&2 Your info is for Adadaptation range 1 and adaptation range 2 (FRA RKAT) for banks 1&2. Your comparing apples to oranges.

Clean the inside of your throttle body.

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Did that last summer and can't imagine that it would be messed up again...

As Blue pointed out, you are trying to compare 2 different things. Might be looking at an issue that isn't really an issue at all.

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Correct. If the car runs fine has good power decent fuel milage, and no codes then the car is performing within design parameters. To look at a screen shot of something like Fuel trim data and think oh this is great because it comes from so and so is pointless. Without surrounds data like coolant temp. RPM, Engine load, Throttle position sensor data, O2 sensor data. That screen shot is meaningless. In most conditions to look at a single frame of data of any type without surrounding data is a waste of time. When looking at fuel trims as described by something like Durametric they are broken down into RPM ranges. Idle, cruise, and upper RPM. IIRC fuel trim or adaptation is figured or calculated differently for each range. So you can't compare fuel trims from idle to fuel trims from cruise or mid range and have it make any sense. Also if your at one RPM say Idle how could you have fuel trim at another RPM at the same time.

to add to that, just because it may "look" different doesn't mean it's bad either. things need to be within tolerances which can be found within the FSM. if it isn't broke, dont look at it. lol