1988 M3 idle speed

[arrowpov]

I have a 88 M3, I am the original owner, it just turned 80,000 miles.
The car is stock, except for a chip from Bavarian auto.

I have a problem keeping the idle consistant. The motor can be at idle around 900 rpm than jump up to 1,800 to 2,000 rpm. If I blip the throttle it may drop the idle speed back down to 900 rpm, but not always. This condition can occur when the car is hot or cold. The idle speed can also drop back down on its own.
The way the idle fluctuates it seems an engine control makes this happen.
Can it be the idle control valve ? The car runs ok except for this problem.
My local dealer has never been good at solving problems for this car.

Thanks in advance if anyone has a solution to this problem.

[m3flyer]

Had the same problem and it was a combination of issues that caused it.

The idle control valve ($25 used, $95 aftermarket to $125 OEM retail - 1 hr. of labor) and was not functioning properly (sometimes there is an air leak at the hoses that connect to this valve; check for this first beore replacing the valve entirely and apply silicone to the junctions before re-assembling), the intake boot was not fastened snugly (ie. - air leaks, you can easily fix this yourself or go to a Valvoline Oil Change place and they will do it for next to nothing), and the intake gasket (very slender oval plenum gasket) needed to be replaced ($17 OE and about 1.5 hrs labor).

Also, you might re-torque the plenum nuts until they are snug ...plus 1/4 turn.

The most common reason for idle issues is air leaks. Start there first.

Best of luck for a speedy resolution...

[az3579]

This is a common problem for all E30's alike. Most of the time it's the idle control valve, but the air leaks, as m3flyer said, is another very possible cause. I've replace my idle control valve already and it still does it, except mine goes to 3000 rpm and doesn't change at all, making it undriveable.

Air leaks, probably. Cracks in the intake boot are a common cause. If you can find an idle control valve for cheap, I actually suggest replacing to see what it does. Start with the free solution; air leak check. Afterward, ICV. If it still does it after checking for leaks and replacing your valve, I have a custom solution that you can easily do yourself, but it probably won't get to that.

[arrowpov]

I replaced most of those rubber hoses this winter, and that did not solve the problem. My first guess was air leaks. The way the idle can increase or decrease quickly makes me think it's that valve.
Thanks