Hunting idle until oil cap removed

[ChattanoogaJSB]

Had another thread going that strayed when I began investigating- decided to appeal for help under a more appropriate title.

Have read some engine swap posts about this problem by this car is stock. New to a friend of mine and Porsche is new to me - though I'm handy and have other classics and a new project 914.

84 911, stock, 127k ish miles. Recent top end rebuild and much maintenance by "specialist" shop. New Bosch o2. I have a Bentley manual.

I'll try to be brief-

Starts easily, runs great under load. Cold idle settles down around 700 rpm but isn't my main presenting problem.

O2 sensor reads normal fluctuations and Intake air valve has been cleaned, is free, has proper resistance and 12v and self-centers both directions with ignition on.

Restart idle is always steady around 850. Idle mixture tamper cover still in place.

Driving the car and coming back to idle at a stop reveals no hunting rpm.

Rev the throttle warm? Idle hunts and will sometimes return, sometimes not. This is the problem! Hunt is always 800-1200 or so. Never more.

Now- remove oil cap the vacuum leak drops idle a tad as expected. Blip the throttle and the idle is rock solid after that. With the oil cap off the car will NOT hunt idle hot, driveability is perfect.

Added an o-ring to old oil cap, no change with the super tight seal. Still prefers the extra unmetered air.

Cleaned pcv beside oil filler- seems functional with perhaps a little blow by, but precious little.

Very confused - have not adjusted idle position switch. Open to any ideas!

Strange the system seems to be able to compensate perfectly for the vacuum leak but not when slightly richer (cap on).

[brianlay]

Make sure the idle switch is working and adjusted correctly. Check the idle mixture. Can't expect the adjustment to last the life if the engine! Apparently too rich at idle.

[ChattanoogaJSB]

Originally Posted by brianlay

Make sure the idle switch is working and adjusted correctly. Check the idle mixture. Can't expect the adjustment to last the life if the engine! Apparently too rich at idle.

Thank you Brian! I'll get after it in the morning.

[old man neri]

Originally Posted by ChattanoogaJSB

Idle mixture tamper cover still in place.

Maybe it's time that cover comes off and someone sets the mixture.