Idle Problem

[Geoff Hoad]

OK, its like this. The car is a 1986 318iM with L-Jetronic (no M-Tronic) with a cataltyic converter. Pretty similar to the '84 and '85 US models except the steering wheel is on the right side. (Sorry, couldn't help myself!). Up to now has driven flawlessly. Started it up the other day and the idle was as rough as you could get. The car stalled and revs dropped to around 400-500 rpm on cold idle. After car warmed up the idle was still low at around 500 rpm.

Attacked it with a timing light and found the timing was off, way off. A bit surprising since the car hadn't shown any symptoms of poor timing. I remember that when I had the car tuned they mentioned they couldn't get a good idle with the O2 sensor connected and disconnected it. After timing the car it ran a lot better. The idle problem was still there but nowhere as bad.

Then I reconnected the O2 sensor. Immediately the idle raised to 1,200 rpm and only came down after about 15 minutes to normal spec (850 rpm). Interestingly the cold idle problem changed quite a bit. Car started as normal went to about 1,000 rpm (normal) then dropped to about 500 rpm and after a few minutes went to 1,200 rpm. Incidentally, the car with the O2 sensor seems to have a lot more power.

Using my trusty Bentley's I went through every test I could find. The Idle Air Stabilizer Valve (mine's the metal variety) showed an incorerct reading, so I replaced it. Idle was smoother and stronger. Went through and tested the Idle Speed Control Unit all all was well. Checked the coolant temperature sensor when cold and found it gave the reading of a hot engine. Haven't replaced it but that's next on the agenda.

So what I have established is that all the signals to the Idle Speed Control Unit are within specification. I have no means of testing the Idle Speed Control Unit itself. But if the idle is changing as noted above then its either working ok or may have some type of mafunction. One other thing. When I turn the air con on the revs drop. The system does not attempt to raise the idle to compensate.

If anyone has any ideas as to why the idle is all over the place I would be very happy to hear from them.

Cheers,

[Unregistered]

I would check long and heard for a vacuum leak, unregulated air can cause some pretty unpleasant idle problems with these cars