78 911 SC idle problems
 

My SC starts well when cold but when it warms up it will not hold a steady idle.
The idle will fluctuate up and down.
It will not die most of the time but on occassion it will.
The car runs great on the highway.
Also, Sometimes it will not start when it is warm.
The P.O. says that it would occasionally do this and he suggested that i spray WD- 40 in the airbox to get it started.
I have had to do this a couple times as it just would not start.
Is it my injectors?
I really need to solve this problem so that i can enjoy the car.
Any idea? Thanks

It sounds like it may be too rich - you could try leaning it out a bit at idle and see if that helps - the adjust screw goes anti clockwise to lean - 3mm allen screw.
Hot start problems often fuel accumulator not holding pressure - but it may be related to a mixture issue.
You would only need to turn the screw 1/8 - 1/4 turn to see if it helps. If not, put it back where it was and you will need to start looking at fuel pressures - lots of threads on that. The mixture screw is down between the fuel head and rubber air hat - again, should be plenty of threads to search.
Alan

I would take Alans advice here.

Hot-start issues are common and well documented here. I would start by replacing the fuel-pump check-valve, its a cheap change. Make sure to get fresh copper o-rings, they wear and leak.

Idle fluctuation sounds like it could be a richness issue. When you rev the engine does it rev up and then fall almost down to 0 before catching itself and returning to idle? My wrench noticed that on my car and said it was a richness issue and to do basically what Alan has mentioned. Lean it out, rev it, listen to idle.. continue to lean it out until it drops revs to the idle state without sounding like its gonna die. Not exactly scientific but that was the advice.

Good luck.

Is this a new 911 to you? You're well advised to take it to a shop that knows the CIS system well and have the %CO set with a proper exhaust gas analyzer. Better yet, get a better baseline tune with new spark plugs, new distributor cap and rotor, CIS fuel pressure checks, valve adjustment, and timing setting and mixture setting with exhaust gas analyzer. All of this you can do yourself except the mixture setting using the exhaust gas analyzer.

Hunting idle is often a sign of rich running condition, but if you haven't baselined everything in a CIS system that is new to you, you're shooting blind.

Brett

NEVER make mixture adjustments until you have checked other things out first because you will just be making a compromise between hot and cold running.

at the minimum fuel pressures need to be checked.

before any mixture adjustments are done, make sure the ignition is in good condition, timing is set and advance works as it should, fuel pressures are good and the hardest to check, NO AIR leaks.

you also need to check for 12v to the warm up regulator (WUR). a simple problem like this could be masked by poor advise of "just adjust the mixture"

your car also has a thermo time valve (TTV) (or it should), if that is bad, it too will make it run rich. the TTV is an elctrical device that blocks vacuum to the WUR when the engine is cold to make it richer for cold startup. if you live where it is warm, you can get away without it, but i like it because it reduces the lean backfire when cold.

The suggestion to try adjusting the mixture was to try and identify the problem. If the mixture is rich, yes, it may be faulty WUR. Or, it may just be a rich mixture. But it is a 30 second shot at isolating the problem. And easlily put back where it was previously if it does not alter the situation. Or, you could spend hours going thru various diagnosis, to find you simply needed a small tweek on the screw.
Alan

The problem is that from the OP's post, it doesn't sound like he is very well-versed on the CIS system, and it sounds like this is a new car to him and that he has little or no history on the past performance of the CIS system or the past maintenance performed on the system.

Brett

+1^
if he starts messing with the mixture and finds it is something else he can fix, he may never get the mixture back to where it was. plus, like you said, it already sounds too rich.

a better way to test it that does not mess up the mixture is to reach in and pull down on the plate in the AFM to lean it out.