Check Valve?

[CedarGrove]

Is there a serviceable check valve at the injection pump? Is there a check valve in the lift pump? My '95 is losing prime overnight if I park facing uphill. Does not seem to be much of a problem if I am facing downhill. Just replaced the fuel filter manager and this helped my problem considerable. Before I replaced that I would lose prime even facing downhill. It is making me crazy.

[DmaxMaverick]

Yes, it has check valves there, and the return line. If you're losing prime, you have leaks, and/or the fuel filler cap is failed. If it doesn't vent properly, fuel contraction will pull air into what may otherwise not be a leak, like QD's and weak seals and connections. This can also cause it to pull air back through the injectors. The only positive pressure in the system should be between the lift pump and injectors, and the only suction should be between the tank and lift pump. The remainder of the system should be neutral pressure, with only gravity playing on it. Leaks lets gravity move fuel where it shouldn't.

[CedarGrove]

Thank you. I suspect the line from the FFM to the injection pump. Going to take the intake off and inspect. I am going to put about 5 psi on the system at the tank fill and look for leaks from there. Any other tips or tricks to find the culprit? When I loosen the fuel tank I do hear a whoosh but it is not clear to me if it is pressure or vacuum. Is this a clue?

[Yukon6.2]

You probably don't need 5 psi.
First try leaving your fuel cap loose overnight in the position where you loose prime.
If that fix's it, Awesome problem solved.
Start with the easy stuffs, a whoosh from the cap is a definite sign to look there

[DmaxMaverick]

The Diesel fuel filler cap vents in both directions, and should not retain noticeable pressure or vacuum. If it does, it's failed, or it's a gasser cap. A bad/wrong cap won't cause leaks, but can exploit weaknesses in the system that may not leak, otherwise. Low air pressure on the system is good for finding leaks, but leaks that occur during vacuum (air intrusion) won't always leak under pressure. Any leak between the lift pump and injection pump should be evident under normal operation, being pressurized, the same as it would if you apply 5 PSI air to the tank. The lift pump should provide 4-10 PSI at engine idle. Applying low air pressure to the tank may show a new leak that is outside that part of the system, such as a return line, which should not normally leak at low/no pressure (during operation), but may if higher pressure is applied. Age, deterioration, and wear can also add to the possibilities.