What causes high fuel pressure?
 

I have a 1990 560 sel, I have replaced the fuel distributor, EHA, fuel pumps, etc, and also done a complete valve job and replaced the timing chain, but the fuel pressure was a bit high, approx. 95 psi. The car runs great for a tank or two, but as the mileage begins to plummet (sometimes within a few miles), the car loses power, burns really rich, and dies. It seems as though the fuel pressure is too high, and also not constant.

That is the correct fuel pressure! IN fact the spec is 6.2 to 6.4 bar.
Computer(ECU) controls the differential pressure & that is real control for fuel mixture. Intermittant sensor input to the ECU can cause erratic fuel mixture.

Sounds like a sticking fuel pressure regulator. Make sure the return line to the fuel tank has not been smashed. Other than that.....??????
Brian

Thanks for the pointers. What makes the car go again is if I hook the fuel distributor up to a Sun MotorVac and run it through the system (engine off) for about 15 minutes. The car then runs fine for a few hundred miles, then very rapidly goes rich again. Has anyone seen this problem?

The fuel consumption increases dramatically when this starts to happen, so it runs really rich until it's put on the MotorVac. So if the fuel system pressure is in spec, what could cause this?

Fuel enrichment on CIS fuel systems isn't accomplished by system pressure BUT by differential pressure. Differential pressure is the difference between the upper part of the fuel distributor VS the lower chamber. ON your car that differential is accomplished both mechanical & enhanched with electronics.

Thanks for the theory, but what's the diagnosis? Has anyone seen this before? Is there contamination in my tank that's fouling the fuel distributor?

Does anyone have any ideas....
 

...to my earlier questions?:confused: