1991 E34 525i M50 uneven idle

[Linuxguru]

Hi,

My tireless companion was last week struck with sudden idle sickness and is not willing to run properly when applying throttle.

I found a youtube video which demonstrates the exact same idle noise as my car produce as far as I can judge:

http://www.youtube.com/watch?v=WVbNG8t-cW0

The following have been observed/performed:

- Idle rev is around 500rpm and constantly struggling. Normally my car idles at 650-700rpm. There is almost no pulsating on the tacometer, although the sound in the video indicate that the engine is pulsating.
- By applying throttle I can get it up to 2k-3k rpm, but by holding the throttle this way the engine will choke itself back to idle after 2-3 seconds.
- By disconnecting the MAF cable, the car runs almost as normal. I can sometime hear the pulsating sound slightly, but the car appears to go normally even when holding throttle over a period of time.
- Disconneced and cleaned MAF, no change. When replugging the MAF, the car goes back to struggling on idle around 500rpm. To me this indicate that the MAF is actually producing a signal, and is most likely not faulty.
- Sprayed all vacuum hoses with start spray + gone over vacuum tubes. Difficult to reach in under manifold, but no change in rev was experienced during this.
- ICV changed about 4-5 years ago, together with inlet and vacuum hoses. When ICV was faulty, the rev was constantly hunting. I don't experience this now, so I dont suspect the ICV.

- What to check next? Can faulty ECT sensor cause this?
- Can I disconnect the MAF cable and drive to mechanic without destroying something?

Thanks in advance for suggestions.

[ahlem]

Does your car have a fuel pressure regulator similar to the M20 motors? If you pinch the fuel hose there does the idle correct?

[Linuxguru]

Thanks for the suggestion, however I have no idea if my car have a fuel pressure regulator or what to look for if it does indeed have one ...

Edit: OK, found pics - will verify and report back ... does it matter which side I will be doin' this pinching? :)

Edit2: Cannot find anything resembling a fuel pressure regulator in my engine room, so perhaps my car doesnt have one?

[genphreak]

The FPR is up the back and is a common culprit- at least the FPR hose deteriorating is :).

They are hard to get to, you need small monkey hands to reach under the manifold to where the pipe connects to the FPR, once the big injector rail cover is removed. The ends of the pipe near the connections are most likely to be perished. Expect to change the pipe anyway, they don't last long. Also, inspect/squish the intake boot and ICV pipes, particularly the connection to the middle of the intake manifold, for cracks and splits and check the fuel pressure if it is not a pipe venting air in.

Oh and beware fragile connectors!

[whiskychaser]

If you havent found it yet, FPR no 6:
http://www.realoem.com/bmw/showparts.do?model=HD68&mospid=47405&btnr=13_0347&hg=13&fg=15
O2 sensor can also affect your idle:
http://www.lambdapower.co.uk/diagnosis/diagnostic_index.asp#q3
Item 8 gives you a list of other items to check too

[bubba966]

Sounds like a sudden large vacuum leak. Main cause of that in M50's is part #9 here
http://www.realoem.com/bmw/showparts.do?model=HD63&mospid=47404&btnr=11_1124&hg=11&fg=40

The retaining clip snaps over time and that piece will work it's way out of the intake manifold. That causes a sudden and significant vacuum leak that will make the engine behave as you describe. Go to the dealer, give them $6 or so for a new one and slap it in. Should fix your problem right quick.

Had pretty much the same thing happen to a friend's '95 525iT. Found that silly connector to be the culprit after weeks of trying to figure it out with much $ spent on new parts that weren't needed (MAF, coils, plugs, etc.). Was annoyed to find it to be a $6 vacuum line connector that breaks so often it's always in stock at the dealer.

[Linuxguru]

Finally found a ruptured vakuum tube going into the FPR, after shortening the tube and reconnecting it to the FPR the car runs without misfiring/choking.

But the idle is still uneven and when testdriving just now the car seemed extremely "lazy" compared to normal behaviour. Disconnecting the MAF, the engine becomes normal and is not "lazy" anymore.

Will probably need to get car diagnosed at a mechanics.

[Grantus]

O2 Sensor?

[Linuxguru]

O2 Sensor?

I was under the impression the ECU didn't use the reading from the O2 sensor directly after startup, but rather waited after the engine warm up first before the ECU uses the O2 sensor reading to adjust the mixture?

My car is acting up from the start.

[genphreak]

Reset the ECU (disconnect the battery for sometime) or drive a long way (up to 500km).

What you have to understand is that running like that, the ECU will adjst all its parameters to fix combustion, but nothing it does works, so the adjustments it will still be trying will be all over the place. Right now it has very little idea how to run your engine properly... Check for air leaks in any pipes and connectors (M50 has many), I replaced all of mine, inc. the O-rings and its worth it as each leak will cost money over time... Get new plugs if they you cannot verifgy the gap is exact- and that they are the right type. See my NGK plug thread for more... then do an ECU reset and see how she.. really G-O-E-S!

[bubba966]

Have you cleaned the ICV? Yeah, it's not easy to get to on the M50. But it's very easy to clean and should be cleaned periodically anyways.

Though it still sounds like either a bad MAF or another vac leak. Would guess the MAF more though since you say unplugging it instantly fixes the problem. There someone around with a known good MAF you can try swapping in to test it out with?