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928S 1984 intermittently not start when warm

  • 1984
  • 928S
70 000
New York

I have hit the snag on restoring 928 which I cant resolve. The engine will not start intermittently when hot and driven for a while. The entire engine top was completely redone. All electronics except alternator is new and properly rebuilt and checked, including every sensor and all fuel injectors and cold start valve. Harness checked for continuity. Timing and CO is properly adjusted. Normally the engine performs and start like new. Once in the while it would start and die.  I feels like not all plugs are firing. Smell of raw gas from exhaust is present. Stepping on accelerator does not help. Eventually it starts to shake and attempts to recover from the stall. Rpms are going slightly up and down until it stabilizes. I have fuel pressure gauge permanently mounted on the fuel rail. While malfunction is in progress,  the fuel pressure is steady at 2.5 bar like in the book. I also have permanently installed vacuum gauge off the fuel pressure regulators line. The gauge is moving and showing the vacuum fluctuates with RPM. My fuel pump is new with filter and fuel tank is washed clean with new screen. Cold start valve is OEM new as well. I disassembled and rebuilt Auxiliary Air valve. It had debris inside. I feel it is a little slow to react to 12V. I'm not sure if the element inside gets weaker with age. 


Any help will be appreciated



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