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Thanks for your reply, was thinking the MAF, but it's often over diagnosed and I don't know anything about Mercedes-Benz as this is the first one I have owned/worked on. I took the STFT and MAF readings at idle, 1500, and 3000rpm, STFT seems to jump to negative only during idle.
Pulled the codes again today, the LTFT and STFT were both -20 and -22, suggesting its over fuelled. Possibly an over reading MAF? My scan tool (torque) shows anything from 7.5-12.6 grams sec at idle, i'd expect this to be more like 4.5-7, but I don't know anything about this engine/manufacturer. Unfortunately, my scan too doesn't show LTFT live.
Anyway, I pulled the airbox and cleaned the MAF. A quick test drive seems to show a more stable STFT at idle, so I guess I will have to see if the fault reoccurs.
dirty air filters can amplify a less- than- clean- maf. i assume you checked them. if in doubt run the truck without filters and observe fuel trims. check air intake entry points as they sometimes tend to get blocked with plastic pieces that move around.
also a "drifted" throttle body blade will result in computer thinking combustion is lean or rich depending on which way it has drifted. throttle body motor and airflow data need to be pulled with das and module re-teached with star/das if needed.
lastly if all of the above checks out you might have developed leaky injectors (yes- on both banks- i have seen it) due to bad fuel. while at at i would also check fuel pressure if it is on the high side (bad fuel pressure regulator).
and truly lastly a semi- plugged cats can result in rich burning condition at idle.
Thanks for the info alx, yes the filters had been replaced recently, and they were clean, in fact, the whole engine bay is spotless. I just recently acquired this car. Will post back what I find.
Was considering fuel pressure and leaking injectors, I thought that would be less likely on a 2013 car, on both banks, never assume I guess. Honestly, though, I have only ever had to deal with lean conditions in the past, on other manufacturers, fuel pumps, vac leaks, etc.. so your advice is helpful.
After realizing that the ODB2 scantool that I use is older than the truck, I upgraded my version of torque to the paid version and re scanned for faults. It returned p0172, p0175, p0746, p0750, p1304, p2092, p2096.
It describes the fault I was seeing (p0746+p0750) & (p2092+p2096), on the right hand side of the flow chart. The 1st check is to see if the seal from the airbox has cracks in it, and check for correct installation of the airbox. I found the clip that holds the airbox in place was incorrectly installed, it was out of the holes on either side of the MAF. I also removed the seal from the airbox, rotated it and reinstalled the seal and airbox correctly.
A test drive now shows LTFT at -0.8 and STFT +/-5, great!. Also the MAF shows around 5.5-6.5 when idling around 550 rpm, which is closer to what i'd expect to see, and the fuel economy has jumped up 15%.
So that's it, the airfilter box was incorrectly installed! I guess the incorrect installation was causing turbulence which resulted in incorrect MAF readings, as I can't see that it would have been restricting airflow to any large extent.
09-21-2016, 06:24 PM
