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We have a 99 E350 in the shop with an intermittent stalling issue. Stalling issue occurs while driving down the road, does not stall at stops. Codes found p0171 and p0174. Scanned data showed maf g/s reading approximately at 3.4-8.5 at idle. Engine rpm remained stable with these readings and no loads were on. Both upstream oxygen sensors were not switching and stuck lean until I unplugged the maf sensor. Once I unplugged the maf upstream oxygen sensors began to switch. I found the maf sensor had been replaced with a aftermarket part and all the clamps on the intake boot were loose. Tightened clamps, cleared codes and issue with fuel trims and maf g/s still present. Verified maf circuit is good. Concluded maf was bad. Replaced maf and STFT now +\-5%. LTFT now +\-3%. These fuel trims are with no loads, engine operating temp, closed loop etc etc. The engine now runs exceptionally better. I noticed with the air conditioning on only at idle the LTFT on both banks is 25%. IAC at idle with a/c on is 42% duty cycle. If you turn the a/c off immediately long term fuel trims normalize +\-3% and IAC VALVE duty cycle is varying but around 30%. Is this normal operation for the LTFT on both banks to be 25% only with a/c on at idle? Thank you
So putting in drive, holding the brake and giving it gas and the fuel trims are okay? If it's only doing it with the A/C on try a capacitor discharge or KAM reset or both for good measure cause that makes no sense.
I think you're seeing the fuel control system switching between trim cells when you're turning the A/C off and on. The +25% with the A/C on is likely a learned value from when there was a MAF/unmetered air issue. What is the short term trim doing when the long term is at +25%?
The PCM will eventually relearn the trims on its own. But, doing a KAM reset like ferris48 suggested will get you faster results.
I did a KAM and global reset with no change. When engine braking LTFT are normal, including if air conditioning is on. While driving fuel trims are normal as well with or without air conditioning on. Only when at idle and the air conditioning is on LTFT are +25% on both banks. You can watch scan data and immediately when you turn the a/c switch off LTFT stabilize at +/-3% on both banks. Watching scan data and immediately when you turn A/C switch on LTFT are +25% on both banks.
Clear KAM again to reset all memory of fuel trims, repeat a/c on/off but keep a keen eye on the upstream o2 sensors. Do they drop lean as soon as the a/c turns on? I want you to see the fuel adaptation happen in real time.
Any sudden changes to maf g/s, baro or ECT with a/c on or off at idle?
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