Need help with a very frustrating bogging/running problem plaguing my 535is. Here are the specifics:
I first noticed the issue after getting gas with the tank nearly empty. Car lost power under acceleration, bucked, stalled, etc. After it idled a while, I was able to get it home. Fiddled with it off and on for months to no avail. Because it sat for periods of time, I would need to jump it. On those occasions, it idled perfectly and ran like a champ. Upon restart, the poor running conditions returned.
I finally gave up and took it to a BMW shop, which replaced the mass air flow meter, plugs and rotor. It ran like new for a few months, but the bogging problem returned.
Essentially, it acts as if the engine is starved of gas, bucking and surging, backfiring sometimes. It does seem to run better on a full tank, but that isn’t always consistent. It also seems sometimes to improve once the engine is warm - on occasion you can feel the transmission grab hold upon letting up on the gas and the problem totally disappears. But not even that is consistent, and lately it starts and idles ok but then starts coughing and shuddering unless you hit the throttle.
I have suspected a weird electrical glitch, but grounds are fine and everything else checks out. Usual switches and sensors appear fine. While it had been running very rich before the MAF replacement, computer now shows 21 MPG, which is very high for this car and certainly doesn’t suggest rich running. Have not had the fuel pump/filter checked, but it seems to be getting gas, compression is good and the BMW shop did not suspect a fuel delivery issue.
Grateful for any ideas. Thanks.