Carrera 3.2 idle issue

[porsche_addict]

I'm having an odd issue with my '87 Carrera. After sitting for a few days and starting the car cold the car idles high at about 1200 rpm and does not come down.

While idling high it also feels rough, almost like a misfire. After a few minutes the idle starts to bounce from 1200 to just under 2000 rpm. If I apply a slight amount of throttle the idle will start to stabilize and return to normal.

If the car doesn't sit for too long the issue does not occur and idles normally.

Is this likely an IAC valve issue or possibly a vacuum leak? I also read about the cylinder head sensor causing similar issues with idle bounce. What should be my first step for diagnosis?

[DRACO A5OG]

Check the OHMS on the CHTS, it could also be loose in the head causing crazy readings.

Doesn't hurt to clean out the ICV too.

[Nick Triesch]

All the above and Techron.

[john walker's workshop]

See if the throttle closed micro-switch is doing it's on/off thing. Check at the plug with an ohmmeter.

[DRACO A5OG]

^^^ this first, the Master knows what he is talking about.

[Tippy]

Originally Posted by john walker's workshop

See if the throttle closed micro-switch is doing it's on/off thing. Check at the plug with an ohmmeter.

^ this

I've used the continuity feature of a multimeter.

If good, vacuum leak is possible.

[DRACO A5OG]

It may be sticky when cold.

[jlex]

Cleaning the idle control valve is an easy thing to do as well....