idle control valve

Accmac · 07-16-2014, 12:04 PM

Hello all,

My first post, I'll try not to embarrass myself. I recently purchased a 1987 911 and had the ac recharged. Shortly afterward the idle started to bounce from about 1000 to 1500 warm or cold. I cleaned and ohmed out the ICV valve and it appears to be in spec. I did notice that if I clamped the upper hose on the ICV that the bouncing stopped immediately and idled correctly. Would that be a clue to my problem? I appreciate any help in this.

Tim Hanson · 07-16-2014, 12:09 PM

Search for mixture and base idle settings... You need to adjust those...look for vacuum leaks first.

Tim H

dan88911 · 07-16-2014, 12:24 PM

Don't be embarrass Accmac. Most people here are happy to help.
Here is post on vacuum leaks Vacuum Leak

There are others try the search tool type in, vacuum leaks

techman1 · techman1's Garage

And here is a brief of the idle circuit:
At idle closed, the idle switch sends a signal to the DME. DME then takes over adjusting the idle by sending a signal to the ICV, the ICV opens or closes, regulating extra air into the engine. If the idle switch was not working, and a big vacuum leak, or a stuck wide open ICV might give a really high idle.
If the idle gets too high, around 1200, the DME reads this, and cuts the signal to the injectors or the spark, I am not sure which. It then restores the signal when the rpm has dropped.

What usually happens - the ICV gets dirty, and the valve is stuck in a position, letting in a certain amount of air. This may be good for a cold idle of 900 rpm, but as the engine warms it is enough to bring it up to 1100, maybe higher. The problem is the DME will cut that signal at around 1200, so if the ICV is open too much, you get a pulsing effect of about 1 second frequency. Not really a miss.

Want to see how this works? Hold the idle switch closed, in the engine bay, and manually blip the throttle. As you get too high, the DME should cut the signal. this is a way to test the throttle closed switch circuit. Mind you, if the DME does not cut the circuit, this does not say the switch is bad. It could be the wiring to the DME or the DME itself.