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.
Yes, there is an in-tank pump and another down the fuel line.
Negative on replacement of pumps/filter.
I had thought it might be a clog in the fuel filter that would dissipate as the fuel line warmed, but that doesn’t explain why it disappeared entirely for a few months and then returned.
Yeah, these intermittent problems are usually a crap-shoot. If these are all original, I’d replace both pumps and the fuel filter. If nothing else, you’ll take these off of the list of potential causes.
Those Bosch pumps can certainly be intermittent when problems occur and just offhand that might be my first suspect.
For what it’s worth anyway, I’ve cut a few suspect pumps open which were replaced on a barely educated guess and all of them had severe wear problems. The armature commutators were badly worn and so was the lower impeller bushings.
BMW was foolish because they did use CIS on certain series and years although I cannot remember which and what. The mid/late 70s best as I can remember and maybe into the early 80s. (???)
According to the VW rep the CIS system is pretty much infallible and never needs a thing; not even adjustments. Ever. Corporate philosophy at it’s finest…
