1996 VP 5.0 fuel pressure loss

[4 Wins]

Details: 1996 Volvo Penta (Ford) 5.0 FI (model 50FAPNCS) with OMC Cobra SX outdrive

Problem. Long cranks before starting mostly when warm, can take 5-8 seconds before starting. If it's being started within 20 minutes of running, it starts immediately. After sitting for 30-90 minutes, long crank (5-8 seconds), and sitting longer like overnight seems to be a bit better but still cranks 2-3 seconds. Idles and runs at speed with no problems.

Troubleshooting:
-Connected fuel pressure gauge with engine cold. Priming fuel pump once showed 7 PSI. Cycled key off and on, and pressure increased to about 20 PSI. Did this several more times and pressure maxed at about 39PSI. Lost 4 PSI in 10 minutes, 9 PSI in 20 minutes.
-Pulled fuel injectors out of manifold still connected to fuel rail and primed fuel pump (this shot one of the injectors out of the fuel rail, used zip ties to hold them all in). Got system up to pressure by cycling key and observed no leaks from injectors.
-Disconnected fuel pressure regulator from return line and put fitting into a bottle to catch fuel. Primed fuel pump and observed strong flow of fuel from regulator return despite fuel pressure starting very low and having to prime pump 4-5 times to get up to pressure. Stream of fuel came out of regulator return each time pump primed. But after priming I didn't see any fuel dripping or leaking from regulator return. Pressure continued to drop.

My questions are

1) is it normal for the fuel pressure regulator to allow a significant amount of fuel through the return when fuel pressure is low
2) If it's not the regulator or injectors, the only other option I can find is the high pressure fuel pump check valve. Any way to test this?

 

[cobalt1999]

Fuel pressure regulator should not be dumping fuel below the pressure specification.

 

[4 Wins]

Thanks Cobalt. I wasn't sure since it only seems to be while the fuel pump is priming. I expected it to keep flowing after the pump shut off (after several cycles) once 40 PSI was reached. May try replacing it.

 

[4 Wins]

I replaced the fuel pressure regulator and am still losing pressure in the fuel rail quickly, about 20 PSI in 10-15 minutes. It's either the high pressure fuel pump or the fuel reservoir. Anyone know how the reservoir works? I believe the regulator and low pressure pump go into it, and the high pressure pump pulls out of it. There is also a vacuum line to it coming from the intake manifold. I don't know how maintains pressure in the high pressure pump/fuel rail/FPR loop...assuming there has to be a valve or something in there.

 

[ripjmk]

The reservoir is the collector of the LP fuel pump discharge. In there is a pressure relief valve that relieves to the suction side of the LP pump at around 10 psi. At the top of the reservoir is a float valve ( a lot like the float in a carb) it releases air and vapors to the inlet manifold and keeps the reservoir full of gasoline. The return fuel from the pressure regulator enters the reservoir in the top and a shaped plastic pipe takes down to the bottom on the other side of a baffle that separates it from the LP relief valve mentioned earlier to avoid short circuiting the cooling jacket that surrounds the reservoir.

Having said all that I have a similar issue with pressure decay and trying to pin it down!

In my instance, I am not getting any fuel return from the pressure regulator when priming. I have ordered an aftermarket HP pump suspecting it allowing fuel to back flow through it. I think, after looking at the mancave mechanic videos, that there is an integral pressure relief around the gear pump that may have paint pa

 

[ripjmk]

I think, after looking at the mancave mechanic videos, that there is an integral pressure relief around the gear pump that may have paint particles stuck in it. If the new pump holds pressure I'll take the pump apart ant find out!

 

[RobRoller]

I just had my reservoir out the other day and there is indeed a float inside like a carb bowl has with a little needle valve similar to a carb in there. Be careful with all the parts to that reservoir as some of them are obsolete. And the o-rings that go on either side of the "banjo" style bolt are very specific size and material makeup and if not will allow a nice spray of fuel. Ask me how I know...