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Mercedes GL500 negative long term fuel trims question

26 Nov 2019 12:35 - 26 Nov 2019 15:53 #35341 by Basil
I have a 2012 year Mercedes GL500 in with engine codes P0172 and P0175 (Mixtures bank 1 and bank 2 rich) . Long term fuel trims are circa negative 25% Short term trims around negative 3% Car drives, runs and idles normally but fuel consumption seems high even for this small truck ;) O2 sensor readings seem OK but I am no expert. Both front and aft sensors are wideband. I intend to remove the MAF sensor and see i I can see deposits, assuming it's a hot wire type, but a physical examination shows the air filter assembly is not secured properly on the MAF assembly, the clips are not fastened correctly. Could this be enough to influence the long term trims? A full throttle blip shows a rapid change in MAF voltage up to a high level. Plugs and air filters are new, fuel filter was changed less than 6000 iles ago.Vehicle is under 40,000 miles total, any guidance as to what to check appreciated. Scanner is Bosch KTS. Thanks! BTW I have been unable to delete these codes with the KTS, I am not sure why... I am new to the KTS and scan tools in general.
Last edit: 26 Nov 2019 15:53 by Basil.

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27 Nov 2019 11:13 #35375 by cj1
Can you post the MAF readings?
Freeze frame data for the error codes?

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27 Nov 2019 18:37 #35390 by scannerjohn
Although a dirty MAF can cause a rich condition, I'm not convinced at this time that is your major problem. Other than high fuel usage, you mention no other driveability problems. With a bad or problematic MAF you should have low power as well. MAF voltage should go higher with increase in RPM. 1 volt at idle to around 4 volts at high RPM.

Graph your RPM, STFT, LTFT and O2's. Note the readings at idle. Raise the RPM to 2500 and watch the data change. If the trims go towards positive, a dirty MAF can be the problem. With total LFTF at neg 28 I think there is more than a dirty MAF here. Something is up with that air filter box... if the MAF is not dirty, make sure the honeycomb structure has not be damaged. The honeycomb distributes the air flow properly, DO NOT TOUCH IT OR ATTEMPT TO FIX IT. You can clean it with sensor cleaning spray or brake cleaner spray. If damaged, you will have to replace it for the MAF to work properly.

Wide band 02's work differently than narrow band. You should have around 3 volts at idle. A lower voltage would indicate a rich condition and a higher voltage would indicate a lean condition. Change is measured in current flow unlike narrow band which is voltage.

Your engine is getting a massive amount of fuel, so the PCM is taking it away, hence the negative trim numbers. Leaking injectors, too much fuel pressure or 02 problems can cause this. Seal that air box and inspect and clean the MAF, then re check your data to see what if anything has changed. Do one thing at a time then re check data. Don't throw parts at the problem. Do a little investigation first. Time does not cost you, parts do.

Get back to us with the new data.

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