Porsche Repair: 944 starting, starter solenoid, fuel situation

Question
Dave,
I have a 1983 944, with 85,000 miles. Well maintained.  The problem is that it starts and runs well. No problem....until I shut it off after driving it until it is warm.  The engine then will turn over, rapidly, but WILL NOT fire until it has set for a while. Time varies from 15-30 minutes, then it will run.
Help!  I can't depend on it to start.  I stalled it in a bank parking lot, and had to sit for a while.  The police came and asked what I was doing!  Very embarrassing!
Thanks, Hank

Answer
Hi Hank,

You have trouble with the cops for sitting still, not speeding...quite unusual for 944 owners!

Your comment that the engine turns over rapidly eliminates the normal cause of this problem...a hot starter/solenoid.  If you're convinced the starter turns just as fast as it does when the car is cold, the next place to look is the fuel situation.  For some reason the fuel is not reaching the injectors, or at least not in sufficient quantity, when the car is hot.

The fuel pump requires a relay to activate it because the pump is so far from the position of the battery.  It needs all 12 volts to work properly.  I suspect increased resistance in the line to the pump...this can occur whenever the wiring or the pump itself is hot from running.  This means less than 12v is getting to the pump.  While you're driving it wouldn't be noticed but when the pump has to re-start the problem emerges.

I can only suggest testing the pump when it's hot and required to re-start if you can duplicate this situation in your driveway) to see if it is getting battery voltage when you try to start the car.  Compare the battery voltage reading when the key is off to the voltage reading coming to the fuel pump when the engine's cranking.  The readings should be very, very close.  If not, check the wiring from the relay to the pump for proximity to heat.

I'm reluctant to say go buy a new pump, because it may be OK, but how old is it anyway?

Dave