1980 five speed manual – periodically stalls

I have an excellent Porsche auto shop near me in Hudson Valley, New York. I put my 928 in the shop earlier this summer with a list of issues to resolve, among them was the car would stall when stopped and the A/C unit was on. The shop found that the idle speed was too low and that was the likely cause of the car stalling when the A/C was on. The shop observed that when the car was started if the lights were turned on or the A/C unit was started the car would stall. However, if the car was started and the engine was revved to about 1500 rpm for just a few seconds and the lights or A/C was subsequently turned on the car would not stall.After all the other work was done except resolving the “stalling issue” I took delivery of the car. I needed to get it back because I had to leave NY for a couple of months, and I did not want to just let the car sit in the shop. After I picked the car up, I drove around for a couple of days with no issues, I was very pleased with the performance of the car. Near the end of the second day, I realized that I had not turned the headlights on (I had new halogen headlights installed). With the car still warm while parked in my garage, I turned the headlights on. They were fine when I turned the headlights off the car stalled. It would not subsequently restart. After four or five attempts I stopped trying. I came back after a half-hour or 45 minutes or so and it started just fine.The next day I called the shop and discussed this latest stalling issue. They told me when it stalled previously it would restart immediately. We’re scratching our heads with no good theories about what could cause the stalling mystery. There could be different issues causing the car to stall in different ways. If anyone has witnessed a similar stalling problem, please share what you have learned.Thanks,Glenn

Does that model have the vacuum adjustable headlights? If so it could be related, or a voltage issue. Check alternator and charging stats.  The headlight motor is pretty powerful, could be taxing the system if it's not grounded properly.

Thanks for your feedback. I won't be able to follow up on the points you made until the spring. I'm pretty sure this model does not have vacuum adjustable headlights; I'll verify this later. We were wondering if part of the wiring harness for the headlights were being pinched somehow. This is in line with your thought about the headlight motor not being grounded. I will include this in my troubleshooting plan. Thank you again for your thoughtfulness.  

Start with the easy stuff as LADEEF said: clean all the grounds. Look for the locations chart/PDF on Rennlist. That usually helps almost everything on the car. Basics before parts is a good rule we've all learned with these cars. Good Luck.

Thank you. I'll look for the locations chart/PDF on Rennlist, if I can't find it I'll reach out.

This is a couple months old, but deserves a little attention for future searhers/readers.

The car includes a solenoid air valve that opens when the A/C compressor is engaged.  Intent is to solve exactly the symptom reported, by allowing a metered amount of air to bypass the throttle and allow more load without stalling.  The valve is mounted on the crossbar in front of the engine.  It has two 15mm fabric-covered rubber hoses that connect to the intake around the throttle. One grabs fresh air, the other side connects to the intake manifold and has vacuum when the engine is running.  The electrical connections include one in parallel/common with the AC compressor clutch, and the other is to chassis ground.

To my eye, the system looks a bit like an afterthought, likely because AC was an option not selected in most markets.  As the US became the major single market, later versions were shipped here with AC as a default option.

What to do:
-- Verify that the solenoid valve is actually present.  It's should be fastened to the crossbar with a rubber-sleeved P-clamp, right of center (right is passenger side of US cars).
-- Verify the electrical connections with a meter.  You should see battery voltage on one pin in the connector while AC is engaged, and chassis ground on the other. 
-- The valve will click as power is applied and removed.

You can find good used valves at 928 International if it turns out that you need a replacement.