Bouncing Idle

So I got the 944 charging this weekend and took it out on its maiden voyage of more than just down the road. All is well car ran very good. About 15 minutes into the drive when I stopped at a stop sign the idle would bounce between 1,000 and 1,500 rpms. Any ideas of where I should start checking? Could it be a vacuum leak, maybe a bad sensor?

For what is worth after the engine has been shut off and turned back on it idles fine again until I go a little distance and stop again.

Posted this in my build thread but figured much more visibility here.

Thanks GRMers

Do those cars have mechanical fuel injection , like Bosch CIS? If it does then the idle compensator is a likely culprit. They tend to get stuck in one position. My VW GTI will change idle speed every second or two. A new compensator is over $400 for my car , so I need to hit the junk yard.

There are three things clarks garage recommends trying so I'm going to start there. Hopefully one of them solves my issue!

I'm not a 944 guy, but I'm betting they have Bosch Motronic. Depending on the version of Motronic, there is a throttle position switch that should be closed when at idle. Check that thing out. Cleaning out the idle control valve wouldn't be a horrible idea, but if that switch is out of whack it will do the same thing on E28s.

Clean the flapper door and the floor and roof with a rag damp with cleaner. I was told DO NOT hose the unit down the rubber seal on the shaft is weak from the factory and the cleaner will eat it. Air leak is common yes. Been a long time since I had my 85- 944 but I miss it often.

Is there a resistance test you can do on the throttle sensor?

I think those cars are Motronic 2.1. I'm a 1.X guy myself, so all those fancy anti-knock sensors and everything are over my head.

Appleseed wrote: Is there a resistance test you can do on the throttle sensor?

yes but its not on the throttle its on the flapper door. It is common on 944 to have a flat spot in it.

Come to think of it I have one somewhere I never needed...

If I'm not mistaken, the 944 (like many other Bosch cars) has an idle stabilizer valve that is prone to getting gunked up. Might be worth a cleaning.

Not too mention the hoses to/from the idle stabilizer can fail.

I've even had the glued in nipple on the intake manifold pop out on my 944 turbo.

That caused a 3000rpm idle, but if it was loose or leaking...

pointofdeparture wrote: If I'm not mistaken, the 944 (like many other Bosch cars) has an idle stabilizer valve that is prone to getting gunked up. Might be worth a cleaning.

This is what I will be trying first. The below is from Clark's garage on how to trouble shoot the area.

Idle Stabilizer / Auxiliary Air Valve

"Clamp line from Idle Stabilizer / Auxiliary Air Valve to the intake manifold to see if idle stabilizes. If it does remove the Idle Stabilizer / Auxiliary Air Valve, clean it with Carburetor and Choke cleaner, reinstall and check operation. If the surging idle persists, replace the Idle Stabilizer / Auxiliary Air Valve."

The throttle bodies can also leak at the shafts and replacement O-rings can help.