NP993

So after three years of ownership I finally have a mechanical problem. For no apparent reason at all today, while driving, my car decided to idle at 2100 RPM's. I drove back home and gave it a visual once-over. Nothing out of order that I could see in the engine bay, all the vacuum lines were in place, etc.

Other than idling at 2100 RPM's it starts, drives, and performs fine. I let it cool down and later in the day went to a friend's place, and once again it was idling high, exactly as before. Hung out at the BBQ for a couple of hours, then left to go home -- and now suddenly it's idling normally. Except that a couple times after taking the car out of gear, the idle would settle a little high, like at 950 RPM's (it normally idles at 500-600 RPM's).

This is very puzzling. The car is mechanically in excellent shape and the only recent work I've performed is removing the engine/gearbox last month to do the clutch, re-gear the transmission, and a bunch of maintenance, but I've put almost a thousand miles on the car since then, all perfectly trouble-free.

When it was idling high, it felt like an old 911 distributor advance problem -- a hanging idle. Is there any reason to think this is a malfunctioning cruise control problem (cruise works fine, I checked it) or a throttle linkage/cable issue? Vacuum leaks usually don't come and go for no reason. I'm stumped.

BobbyT

You may want to clean the idle stabilizer valve. Here is how:

http://www.pcarworkshop.com/index.ph...-_ISV_Cleaning

You might also cycle the throttle linkage while the engine is off, from the driver's seat and from the engine bay, to make sure it moves freely without any binding or sticking.

I'd put the cruise control pretty low on the suspect list. The usual failure mode is unable to engage or hold set speed.

JM993

All good suggestions above.

I just had this issue on my old 325is. It was the throttle position switch. 993s have them too so you may want to check it!

Cheers,
Joe

Terry Adams

I thought mine was the cc because it was engaged when it happened. It was the ISV. Dealer thought it was a cable problem, drop engine for over $1K to check it out.

Also vac under the carpet near the gas pedal. There is a switch there.

NP993

Thanks everyone. I cleaned the ISV when the motor was out. Do they malfunction and cause an intermittent high idle?

HAKOSUKA

just wonder if you found the problem? i have the same problem as urs and its not always its just some days it wants to mess around with me

stevepeck1

I've seen a dozen threads on this and few which reply and said what the problem turned out to be - anyone? Chasing this gremlin presently. Intermittent...

IainM

Does anyone have any codes?

Cactus

My car starts at a higher rpm when warm and nobody was ever able to diagnose it. My ISV valve couldn't be any cleaner.

IainM

My guess would be a sensor intermittent of some sort. But most will throw a code, hence my question ^

these 25yr old cars suffer from contact corrosion and taking the engine out and back in can jiggle connectors into non contact.
solution is easy, disconnect, squirt with contact cleaner (carb cleaner at a pinch) and then make-break-make the connectors a few times to ensure good contact. Then you’re good for the next 25yrs.

a good scan tool will get you to the offending plug quicker but likely they could all use a clean