Sporadic Idle After ~20 Minutes

[83GR944]

I have an '83 944 that idles fine at 1k RPMs when cold, but after driving for about 20 minutes, the idle will rev up to about 1,200 and then drop to around 800, causing the car to shudder. This happens over and over again until I shut the car off and it cools down.

I had a cracked intake manifold gasket but replaced that, and I just cleaned the throttle body, but neither helped this specific problem.

Any help would be appreciated.

[rporzio]

I had a problem just like that with my 84 and cured it with making a good seal with RTV all around the cover to my AFM. It was cracked in several places and loose all around. Advance Auto helped me with a small tube of black RTV and my pulsating idle was gone...

[mikepellegrini]

It sounds like a vacuum leak.

I had a similar problem with my 83 NA and it drove me nuts for several months.

I fixed it by adjusting the large screw on the throttle body. The manual calls it a "regulating screw" or "by-pass screw." I'm not sure if it's an idle jet adjustment or if it adjusts the vacuum. Butat least for me with a similar problem, it worked.

The car had been idling at around 1,100 RPM (varying between that and 800 or so after warm); after adjusting the screw, it now idles about 1,000 - and no surging.

Weird cars...

[83GR944]

I have already opened up the AFM to adjust the wiper arm track and made sure to use plenty of RTV when I sealed it back up.

Thanks for the suggestion though.

I will look for a leak and see what happens from there.

Thanks for your help.