Mine (83 k-jet) did similar recently. Wait 5-10 mins, would fire up, but in a small time window, nothing. In the end it seemed to be the warm start solenoid temp switch, which is either NLA or silly money. Adapted a 944 part in, and seemed to fix it, but the failure occurred again recently....waited 5mins, ok again, but thats first time in several months now. This is a temp sensor screwed into the hot water outlet fitting that feeds the heater. To test, remove, check open circuit cold, then heat to over 50C, check closed circuit, near zero ohms. Mine was showing 20 ohms, which is enough to stop the solenoid firing.
jp 83 Euro S AT 52k
Fuel pump check valve,
Leaking fuel accumulator,
Those are the two most commmon sources for CIS hot start problems
With those 2 things not working system pressure drops extremely fast and creates a vapor lock in the cis dizzy.
Purchase a CIS system tester to confirm this is the problem, they are around $60.
Accumulator are about $130 give or take a bit.
Or another check is with car hot and not wanting to start jumper the fp relay and run the pump for around a min. As well you can put a momentary switch with 12v to the cold start valve and hold the switch when you start.
1981 928S Euro,
1987 928S4 Auto
1987 928S4 5 speed AFCO double adjustable inverted 1000/500, Stage II w/ tri-flow cams, Aluminum flywheel, dual disc Stage IV Spec clutch, Crankcase vacuum pump, CT4 6767 custom mid mount turbo @ 5.8PSI
PM me for services/parts.
It is a 1978 928. When he shuts it off when hot it will crank but not fire. He can get it to fire with a squirt of starter fluid but it will not stay running. After it cools down and it is cold then it fires right up.
Occasionally my 1985 928 S won't start when it's hot. After reading the threads, I guess I have to start by replacing the fuel pump check valve. In the meantime, does anyone know if there is a way to get the car started when it won't start hot? It cranks but won't fire. I know I'm going to get stuck somewhere and end up waiting a hour or two for it to cool off.
Check the battery terminals, clean and the ground. Mine did the same thing and the ground strap was no good. Car started great cold, not hot, not at all. it would crank for ever. Worth a shot, easy to get at and re do...
My 84 had a failing fuel pump relay. It sits basically over top of the header on the passenger side and was getting warm. When the car was hot the relay couldn't latch and the fuel pump wouldn't come on. Exactly the same result you have.