Rough lumpy idle on 1987 NA
 

I'm trying to chase down the cause of a rough and lumpy idle on my 1987 NA. The tach indicates that it's idling steady at the correct RPM, 900, but it feels and sounds like it's surging -- going "rrrr, rrrr, rrrr, rrrr" instead of staying put. It also vibrates a lot, but that's probably due to worn motor mounts (which I just ordered replacements for).

I checked all of the vacuum lines and they're connected properly. They are original, though, so it's possible there are some small cracks in them I can't see.

Plugs, wires, distributor cap and rotor are all new, so I don't think they're related. Reference sensors are new too. Could they be maladjusted?

Could it be the idle control valve? I thought a failing ICV caused a high idle, but I may be wrong.

Is this a timing issue? Should I check the timing belt adjustment?

Thanks

Probably one of the hoses under the intake has rotted.

You could do the "intake smoke test" and find one of the leaks but the 30 year old engine should have all the hoses and vacuum line connections and plastic hoses under the intake inspected or replaced.

Before spending money on the idle stabilizer clean it carefully inside and make sure the internal pivot valve moves freely.

Check the belts, flush the injectors and replace the screen and seals.

Common problem now and many threads for more info...

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/985354-924s-runing-rough-idle.html

GL
J_AZ