W140 S320 Hard Starting
 

'92 S320 - M104 motor.

Rarely, car starts with barely at touch to the ignition switch.

Usually, you have to crank and crank and crank - probably 10 seconds to get the engine to catch and run.

Fuel pressure at key-on jumps to about 55psi. Checking the specs with the engine running, we get over 60 with the vacuum disconnected from the fuel pressure regulator, and 45 or so with the vacuum attached.

Vacuum hose at regulator is free of any fuel.

Cut engine off - specs say 30 minutes minimum should retain 36psi. Values drop to zero PSI within 2 minutes.

I don't believe these cars have a pressure accumulator, at least I couldn't find one on EPC. So where is/are the check valves that should retain pressure in the system?

What's mysterious to me is that we've got spec level pressure instantly when you turn the key to position 2.

Once the engine starts, it purrs like a kitten. Almost smooth enough to pass the Andy Granatelli quarter on the radiator cap test...

Once running, performance is just fine. Plenty of power (for a 228hp motor in a 5,000# car).

Don't want to throw parts at it - but it needs to start better than it does. I fear it will some day refuse to start at all.

Input is appreciated.

Thanks!

Either the fuel pump check valve or the fuel pressure regulator would be the culprit.