911SC stall problems! Please help!

[Surge]

Hello all, I have a problem with my 78 Porsche 911SC. I've taken the car to numerous shops all of which are incompetent and cannot figure out what the problem is. I am getting desperate and was hoping someone might point me in the right direction.

Symptoms:
When cold, the car goes through the warm up routine (auxiliary air regulator) and idles high. Then once it is done with that routine, the car will stall if I am driving at about 3000+ RPM and then drop it in to neutral and close the throttle. The RPMs will drop below the idle RPM (900) and eventually stall and the red light will turn on (what does that light mean? It has a G on it?)

When warm (after 10-15 minutes), the car does not stall anymore but if I drive above 3000 RPM and then put it in to neutral and close the throttle (same conditions as stated above), the car will still drop below th idle RPM (900) to about 500 or 600 and then "kick" back up again, oscillate once or twice and finally settle around 900 RPM.

In summary, regardless of whether the motor is cold or hot, when you go from driving RPM to cutting all throttle, the RPMs drop far below the idle RPM and the car either stalls or almost stalls. I have increased the idle RPM to 1100 to prevent the car from being undrivable.

Obviously this means the problem exists regardless of whether the car is warm or cold; however, when the car is cold the problem is amplified and the car stalls as a result.

Things I have done in attempts to fix this problem:

1. Replace auxiliary air regulator
2. Replace warm up regulator
3. Remove, clean and reseal throttle body
4. Complete ignition tune up including rotor, cap, wires and spark plugs (by the way, the old spark plugs looked great, they were not fouled up and did not have any signs of excessive lean or rich conditions).

Any suggestions? Thank you for your time.

[JBO]

Mine did something similar, and it was corrected by adjusting the mixture and then idle speed.

[Nostril Cheese]

You might also want to check the rubber boot going from the throttle body to the airbox for cracks. As well as check the airbox itself for cracks if you havent already installed a pop-off valve

[Paulporsche]

I'm w/ JBO. Sounds like your mixture is set too rich. Try leaning it (or having a shop set it) and get back to us.

[vash]

hey, mine did that too. strangely enough, when i took it to the mechanic to set the mixture, he said it was sooo lean that he was surprised i even made it to his shop.

btw, he charged me $40. way cheaper than tossing parts at it.

[Surge]

Hey thanks for all of your replies. Just so that you guys don't think I'm some idiot who just started replacing random parts, yes I took it to a VERY reputable Porsche shop that has "expensive equipment" that takes 1+ hours just to hook up my car. It logs a bunch of different air pressure / fuel pressures and then tells you what the problem is. So far I've taken it to 2 different Porsche specialists and those were their diagnoses so I replaced the parts they told me to replace. Basically they are incompetent.

Oh yeah! I forget, I also had the entire intake manifold ripped off, and resealed. New gaskets, new fuel injectors, new seals everywhere. No progress and VERY expensive! This is all because reputable shops have told me that these were the problems.

I will for sure bring it in and have somone adjust the fuel mixture / idle speed and timing to spec and then repost the results. Thanks guys, I really appreciate the time you spend to help me out.

[oneblueyedog]

Indeed it sounds like the same problem I had for many years. It was mixture a la fuel distributor. It takes a lot of studying but take a look at Souk's thread on:

CIS Idle Speed and Mixture Setting Without an Analyzer

Once I made sure the pressures were correct to the WUR. I was able to set my mixture correctly by adjusting the fuel distributor and idle speed screw respectively, all by how a sensor plate lift affected the engine. They both work together to get the mixture right. That was something I was ignoring-for years.

As said above, you do indeed seem to be rich.

[Hladun]

What you describe is what the deceleration valve is there to prevent. In the screwing around with these systems it’s often removed or incorrectly adjusted.

Of course the CO setting and fuel pressures must be correct. Set with gauges not the ¼ turn method.

Someone should really start a business recycling all those good WUR’s you guys keep throwing out!

[schoward]

Mine did same thing.

Mechanic fine tuned the timing and idle speed and now it is perfect. It took a little trial and error they said - especially factoring in A/C operation. It cost me $ for the labor time but worth it.

Get it corrected. In my opinion that is dangerous. Think im overreacting -- my car decided to stall as i was approaching a yield sign/intersection. my warning lights came on with the stall and distracted me for a split second causing me to glance at guages. WHAM - rear ended the guy in front of me.

[Surge]

Firstly I would like to thank everyone for responding. All of you are awesome.

All you guys that said the mixture was too rich were dead on. I did a ton of research on the CIS system. I studied each component in detail, and tested a few components like the decel valve and the thermo valve. Those turned out to be fine. I then walked through the idle setup procedures and tuned that damn thing myself with no instruments.

I used to build custom turbo kits and tune cars so I know the process and I can hear and smell if the engine is lean/rich. What threw me off to begin with was that the spark plugs looked great so I never realized that the idle setting could be so dramatically off when the rest of the RPM range is fine.

Using your advice that the mixture was too rich I turned down the idle air screw and the idle fuel screw. Working with each screw I found the proper idle speed and approximate fuel setting. The car runs perfect now and it hasn't stalled once since I set it up. It doesn't oscillate either.

I'm going to take it in and supervise someone tuning it to the CO spec and making sure it runs perfect, but in the meantime just wanted to say thanks!