My 1979 Porsche 928 Euro track car has the original Bosch K-Jet CIS fuel injection. Lately it has a hard start when warm problem.
A special fuel pressure gauge set up is needed to diagnose CIS fuel injection. The problem was identified quickly, the fuel pressure was quickly falling after the engine was turned off. There were no visible fuel leaks, so the possible sources of the problem are:
a) fuel pump check valve: a fuel pressure gauge was directly attached to the output of the fuel pump and confirmed that it was holding pressure.
b) fuel pressure accumulator, a device (located in the right rear wheel well) that has a spring loaded plunger to maintain fuel pressure. The accumulator has a diaphragm that can leak. If it leaks, there is a screw capped fitting. If no fuel is coming from that capped fitting, there is no leak there.
c) O-ring seal in the fuel distributor fuel pressure regulator needle. The fuel distributor control pressure is regulated by this device, using spacers to position the rod. If the O-ring fails then system fuel pressure will fall.
It was item C that failed in my 928.
If you need a rebuild of your CIS fuel distributor, I do recommend the services of Larry Fletcher of CIS Flowtech. www.cisflowtech.com
He kindly assisted me with this diagnosis and previous problems.