Question: Idle at Park and at Drive

[MARCUS VETTE]

Hi

Can someone tell me why my idle at drive position are OK but when at (P) parking position, the idle is rough, (up and down), exactly like a leak vacuum.

What can cause this difference? or what's happening at parking position that would create a vacuum.

Marc

 

[john6277]

rough idle - vacuum leaks

For what it is worth, my 95 coupe changed so much for the better when I accidently found a vacuum leak at the EGR valve fitting. The rubber angle fitting in the hose at the EGR was rotted, loose, and would leak vacuum intermittently. Since replacing the vacuum hose fitting it changed the rough idle, acceleration, and hesitation by quite a bit. It may sound crazy, but the key chime reminder even now has the original full strength sound. When the vacuum leak existed, volume of the chime was about 1/2. Go figure. I do not own a vacuum pump tester yet, but this whole area of the engine is too important.

 

[hobbymec]

Hi. I had the same problem in my 89 black c4. Change the IAC- idle control valve, and you will be fine.

 

[Hib Halverson]

Did you check for vacuum leaks?

If you didn't find any, you can approach the problem two ways: 1) diagnose a problem with idle stability using the information in the Service Manual or 2) you can just start replacing parts, in which case I'd start by removing the throttle body, cleaning it, including the IAC bypass then replacing the IAC.