1979 911 SC stalling at idle

[skime219]

112,000 miles
When car has run several miles and fully up to temp it stalls from idle.
Immediately restarts, may not stall at next stop but randomly stalls at future stops.
Seems to be getting worse...mechanic says idle valve?
Ran high end fuel cleaner thru a tank and burning Chevron premium now, still having issue.
All suggestions appreciated.

[universeman]

SC has no idle valve (that's Carrera)

Could be AAR or AAV or (less likely) WUR. Does it idle really low? Have you tried adjusting the idle speed / air bypass screw?

AAR may not be closing. This is what I'm currently dealing with....interested to see what others say.

[skime219]

Idle speed is in the 950 to 1000--believe that is standard
If I hold the idle to 1500 it does not stall

[Paulporsche]

Make sure all your ignition components are up to scratch.

What type of CDI box do you have? Others have reported certain Permatunes failing once they get hot.

Then check your fuel and control pressures. It's possible your warm control pressure is too low, causing a too rich mixture. Often a too rich mix will cause idle hunting and/or a sudden drop in revs when coming to a stop.

Also check for vacuum leaks and make sure your decel valve is operating correctly.

[gazzerr]

I'd say you have a bunch of vacuum leaks and that they have collectively gotten big enough now that the car stalls out. When you drive along and put in the clutch do the revs drop fast, almost stall then recovers but sometimes stalls like at a stop?

Mine was doing the same. I had a huge hole in the black CIS rubber hat that sits on top of throttle body. All my injector seats and o-rings were leaking. Same for the intake gaskets. Also the oil tank level sender gasket. You just have to go through all of it and plug the leaks by renewing hoses, gaskets, seals and o-rings. It's a good bet if your air box is old enough it leaks too in the classic spots.

Here's a great place to start:

Finding CIS vacuum leaks - the vacuum cleaner pressure test

also

Air leakage from intake manifold sleeves

G.

[boyt911sc]

Skime,

Does the car start on the first attempt in the morning (engine stone cold)? If not, how many attempts before it starts to run? When the engine starts, what is the initial engine speed (RPM)? Test or measure your fuel pressures (control and system). At this point, do not attempt to tinker or adjust the mixture setting until you have tested and confirmed the absence of vacuum leak source/s.

Lastly, find a mechanic who is familiar with CIS. The older mechanics seem to be more familiar with CIS than the younger generation of Porsche mechanics. I personally know a few good Porsche mechanics working on the latest models like GT3, etc. but have no clue how to troubleshoot the CIS engines.

Tony

[skime219]

Guys, all excellent advise.
The car cranks no problem with a smooth idle when cold, only have the issue after running several miles, like a good 45 min.
Will start with vacuum leaks and hopefully that will be the cure.
All advise appreciated.

[gazzerr]

One thing I forgot to mention, when the car is warm remove the cap on the oil tank. If the revs drop slightly then you don't have a significant air leak. If nothing happens then you do have a leak.

[skime219]

No drop in rpm after repeated tests.
Guess I go about plugging some leaks...thanks gazzer

[T77911S]

does it surge at idle.

when you let off the gas, does the RPM drop below idle then come back up

[skime219]

no surge at idle, but when i let off the gas it does drop below idle rpm and then come back up
this looks like a doable task just do not have the spare time at this point, work keeps getting in the way of the important stuff
guess i will have to set my priorities
I see gazzer is in GA...maybe I can bribe him to help..

[79 Euro]

Great advise and links. Thanks Gazzerr!

[gazzerr]

Sure no probs. Also take Tony's advice and check your fuel pressures.

[T77911S]

drop in RPM when letting off gas is a sign of rich

[prschmn]

You could try making a manual mixture adjustment and get an idea if it is too lean-maybe from vacuum leaks or too rich. See if you can improve it that way and use that to decide whether you need to be checking pressures or looking for leaks.