Fuel pressure

[amazon69]

Evening.

Another query regarding my old W111. I have replaced all the fuel lines, pump, filters and sorted all the injectors, seals, etc. Car will not start unless I build up fuel pressure by turning key on and off a dozen times or more, then it's fine. I suppose it is not holding fuel pressure. I have not replaced the FPR, not least because I can't find a new one. Maybe there's an aftermarket one that would work (Like this one?). Is there a one way check valve in the fuel system? I didn't see one when I went through the fuel lines. I suppose the obvious thing to do is fit a pressure gauge and watch it. I imagine a well sorted system would hold the 2 bar pressure for weeks on end. Is the FPR a likely culprit? Cheers.
Dave

[Benz Dr.]

Mechanical fuel injection will run on 10 PSI but works better at 15 PSI. There is a check valve right at the outlet on the fuel pump and a fuel pressure regulator on the IP. This system doesn't hold fuel pressure all that long after shut down.

I would rig a fuel pressure gauge right at the cold start valve and see what you have while running. Anything less than 10 PSI and you could have running problems. A good fuel pump will produce one liter of fuel in 15 seconds in terms of volume and 15 PSI in terms of pressure. You can have a bit less than those optimum numbers and still be OK.

Do a volume test after the fuel damper in the engine bay where the fuel line runs back to the tank. I would also check the return line going into and through the fuel tank. Any obstructions will cause all sorts of driving problems after the engine is warmed up. I use a shot of compressed air into the return line to test for blockage - remove the gas cap and use a rag around your air wand so fuel can't blow back at you. If it isn't plugged in the lines you will be able to hear bubbles in the fuel tank.

[Tomguy]

Based on your other thread I am led to believe this is a D-Jet m116 engined car, correct? The 3.5 V8?

If so, the pressure is held by the fuel pump's check valve. It may be dirty causing it to remain open or it may be failing. The pumps are quite expensive new, and there are aftermarket replacements that you can use that fit and provide the required volume and pressure.

Make 100% sure you have no leaks. That'd include not just lines but injectors and the cold start valve. If you're sure nothing "Downstream" from the pump is leaking you may wish to take the pump off and apart to clean it or have a good shop service it versus replacement.

[amazon69]

Thanks. I will check again for leaks. I have a new pump, so hopefully the check valve is ok. Is it possible for the fpr to leak back into the tank?

[Tomguy]

The D-Jet system is not like modern returnless EFI. There's a return line. So if the fuel pressure regulator isn't holding pressure it can cause a leak back to the tank. I am unfamiliar with its internal structure or failure modes to say if it's common or plausible.

[amazon69]

It was that return line I was thinking of. I will get testing, thanks.