Fluctuating revs at idle

Post Fri Jun 01, 2012 4:04 pm

The cut pulsates between 600-1000 RMP at idle. Sometimes gets better once warmed up, sometimes doesn't. Is worse in urban driving (stop-start).

Is it the ICV, the TPS, or a mixture of both? How can I check?

Post Fri Jun 01, 2012 4:12 pm

Did it idle at around 800rpm before this fault developed?

Post Fri Jun 01, 2012 4:42 pm

We ought to bear in mind, this car also has a battery drain. The two things can be related.

Battery Drains:

http://www.e30zone.net/e30zonewiki/inde ... tery_Drain

Rough Idle:

http://www.e30zone.net/e30zonewiki/inde ... ake#Idling

I'd find the battery drain first and remedy it, since it'll be killing the ECU and losing your idle settings. Once the power issue is sorted, then you can look for idle faults, if any still exist.

Post Fri Jun 01, 2012 4:58 pm

DHFiS wrote:Did it idle at around 800rpm before this fault developed?

Yes, give or take.

Grrrmachine wrote:We ought to bear in mind, this car also has a battery drain. The two things can be related.

Battery Drains:

http://www.e30zone.net/e30zonewiki/inde ... tery_Drain

Rough Idle:

http://www.e30zone.net/e30zonewiki/inde ... ake#Idling

I'd find the battery drain first and remedy it, since it'll be killing the ECU and losing your idle settings. Once the power issue is sorted, then you can look for idle faults, if any still exist.

Good point - I hadn't thought of this... Will look into the links, thanks.

Post Sat Jun 02, 2012 4:52 pm

If not electrically related, check & eliminate air leaks. I did something similar to that shown in the link below with generic hoses for peanuts, although the main leak was in the braided hose to the brake servo.
http://www.m42club.com/forums/showthread.php?t=2742

Post Sat Jun 02, 2012 7:01 pm

cee wrote:
DHFiS wrote:Did it idle at around 800rpm before this fault developed?
Yes, give or take.

I would look for a split hose/air leak, at this stage dont make any adjustments to the AFM or throttle stop.

Post Sat Jun 02, 2012 7:08 pm

All breathers/hoses were replaced over the winter with brand new BMW parts

Post Sat Jun 02, 2012 9:08 pm

inlet manifold gaskets also go hard and split.

Post Sat Jun 02, 2012 9:15 pm

When the engines running take the oil cap off if the revs drop theres no air leak.

Post Tue Jun 05, 2012 11:27 pm

sweep wrote:When the engines running take the oil cap off if the revs drop theres no air leak.

Revs drop, and hand gets covered in splatters of oil