M50TUB25 Bogging and Surging Problem

[MaxBell]

My problem doesn't sound anything like a worn out VANOS. What about oil viscosity would make a car bog and stumble in its idle?

I replaced the fuel pump today and still nothing.

Does anyone know what the ground points on the car are? The main ones that could get corroded and cause problems?

[FredK]

I am thinking maybe you have a bad throttle position sensor on your throttle body or a bad DME.


The main ground point is the one on the frame rail in the engine compartment.

[MaxBell]

The TPS and the DME have been replaced already.

I'll go double check that engine ground.

EDIT: Would it be a good idea to start running through the wiring harness? Is there much on the body side that could be affecting the engine?

[scottinAZ]

this may or may not be of assistance, but here it is anyways....

mine turned out to be a combination of a bad knock sensor, dying lift pump and a vanos so worn out that it was not operating in a rational manner any longer. The main problem was for sure the knock sensors, as a true scan of the system with a proper scan tool showed that the rear one was intermittently faulting out. this did not show on a stomp test. The lift pump crapped out on the way home from the scan, as I literally ran out with about 1/4 tank still left... the Vanos reman was planned from before, and what the result of this was is a general smoothing of the operation of the engine. runs and idles like a top now, with no hesitation or other issues. the old vanos that was removed will "self actuate" by nothing more than gravity if held either face up or face down, showing the seals are badly shot. all in all, it was a combination of issues that had me chasing my tail needlessly.

[MaxBell]

Thanks for the reply Scott! I figured it out tho!

SOLUTION: The common ground on the 325e fuel pump was cracked, so when the car went over bumps or when the strap moved, it would lose connection and shut off.

This produced a seemingly random bogging and loss of fuel.

The issue was greatly improved by replacing the fuel pump, but not solved.

From there I rechecked all the grounds on the car and found that the engine ground from the mount arm to the chassis was rusted. I sanded it down and reattached it.

There have been no problems so far!

this may or may not be of assistance, but here it is anyways....

mine turned out to be a combination of a bad knock sensor, dying lift pump and a vanos so worn out that it was not operating in a rational manner any longer. The main problem was for sure the knock sensors, as a true scan of the system with a proper scan tool showed that the rear one was intermittently faulting out. this did not show on a stomp test. The lift pump crapped out on the way home from the scan, as I literally ran out with about 1/4 tank still left... the Vanos reman was planned from before, and what the result of this was is a general smoothing of the operation of the engine. runs and idles like a top now, with no hesitation or other issues. the old vanos that was removed will "self actuate" by nothing more than gravity if held either face up or face down, showing the seals are badly shot. all in all, it was a combination of issues that had me chasing my tail needlessly.

[scottinAZ]

cool, glad you got it sorted.

mine may have been the same issue with that damn in tank ground for the lift pump, but I just replaced the damn thing. 20+years of hard graft, it was most likely shot anyways.....