Porsche Repair: Porsche 944 Starting Problems, jetronic fuel injection, typical cause

Question
Hello Dave,
I read through your past answers to the problems with 944's starting hard, but I still can't seem to figure my problem out. I will let my '84 non-turbo 944 sit out all night, and in the morning it will start just fine. But after running the car for say more than 5 minutes it will not want to start. After letting it sit for another 3 or 4 hours it usually starts up right away, but until that time after being run it will not start. I have been looking the car over and checking the basics, but still can't seem to figure it out. Any help would be fantastic at this point. Thanks,
Evan

Answer
Hi Evan,

I suspect there is a problem with the L-Jetronic fuel injection you have.  I will assume your ignition system is new or nearly new because you've probably replaced all that stuff trying to solve this problem.

One typical cause of your hot start problem is insufficient "residual fuel pressure".  

For cold starts (all those examples where you've waited overnight or three or four hours are definitely cold starts) the engineers built in all kinds of helpful features to get past the engine's initial high craving for fuel.  

For hot starts (let's define that as any starts after only being shut off for 20 minutes or less), the engineers designed the system to maintain a "residual fuel pressure" for 20 minutes after shut off.  Thus, when you turn the key for a hot start, the fuel injectors already have pressurized fuel ready and waiting...and they are not waiting for the fuel pump to deliver it from the tank.

Now...if your residual fuel pressure is not maintained, there's no hot start fuel to help you start.  To test all this you'll need a pressure gauge and fittings to connect to the coupling at your flexible fuel line near the fuel rail.

If the test reveals this problem, the injectors or cold start valve could be leaking, the pressure regulator itself could be faulty, or the fuel pump's check valve could be faulty.  So many possibilities, but the culprit can be found.  

Sorry, I don't have the specs for a 944 in front of me, but generally speaking, I'd like to see 2.5 bar system pressure and 1.5 bar residual pressure for 20 minutes.  Those are the kind of numbers you'll want to see.

Good luck.  If you need more info on doing the test, please write back to me.

Dave