Symptoms of a failing lift pump?

[acme66]

I am getting intermittent power loss. Sometimes I go days between episodes. Trust me when I say I have checked and eliminated all the easy stuff. Solenoid, tank, filter, lines, contamination you name it. I am either down to something like a plastic bag that drifts over an sometimes blocks the pickup or a lift pump that is failing intermittently. I have checked that tank and it looks like a stainless steel bowl. I fished around in it with barbed wire and a spring grabber but have come up with nothing. Once in the middle of an episode I put air to the tank and could only get a dribble out at 3psi of air pressure. But then I pull the pickup tube and.... nothing. All the lines are copper and clear. I haven't run wire through them but they blow out. Like I said I have been through every component and have found no faults. Now I am starting to doubt myself and the time I pressured the tank and wondering if it could have been the pump the whole time. Every one of the mechanics I have talked to feels that if the pump is failing then it fails. Loss of pressure and loss of power. None of this on and off stuff. Anyone here have a lift pump fail and come back then fail again running normally between times? Serious question because I am getting a little crazy out here.

Ken

 

[acme66]

Sorry I left out that it is a 250 in a 1983 M925A1

Ken

 

[WillWagner]

1983 would make it an NHC, correct? No lift pump on those.

 

[acme66]

I think the manual calls it a PT pump. I am using lift pump in a more general sense in that it lifts fuel to the manual injectors. While I appreciate the vocabulary correction can you offer insight as to the issue? I remain stumped.

Ken

 

[73m819]

Does this happen about at the same fuel level ???

 

[acme66]

Most of the time the tank was about half full. But it has happened when I was nearly empty and when nearly full. The issues started the day after I ran the truck low enough it started to surge on the rough part of the tour so I think that may have provoked the issue.

Ken

 

[73m819]

Look for a small pin hole in the in tank pick up tube at about the 1/2 full mark, a small pin hole will suck air, and if small enough will seal itself then suck air, ect. thought the pickup tubes were steel.

 

[acme66]

Mine is copper and I didn't see anything wrong with it when I had it out but I never put pressure to it to check for leaks. I will do that as it is one area where I could see this intermittent power loss. I also was researching last night and realized I haven't checked the fuel return lines either. Ever hear of the governor weights acting up at lower RPMs? Still searceand scratching. It came up the hill this morning fine so fingers crossed. Keeping the tanks topped off each night and have shortened the pickup the by two inches. If something is in the tank I hope that will keep it away until I can solve this then I can replace the pickup.

Ken

 

[Floridianson]

As said check /replace all rubber feed fuel lines. Sounds like you checked in tank pick up. Check fuel filter and if the o ring is good and the bolt plate is seated correct on top of the canister. Would seem if your return lines were closing down it would raise the rail pressure and run better but good new rubber everywhere is a good thing. If you have a gage that went to 200psi and a long line you could watch your pressures as you drive and see if it is dropping off to soon. Loosing my memory but was there not a check valve on the return line up by the PT. Don't know if that hung up when it is needing to hold if that would cause intermittent running or stumbling when running with load.

 

[WillWagner]

The PT pump works or it doesn't. On normal automotive PT systems there is an AFC part of the pump. It controls fuel delivery on acceleration by limiting rail pressure with manifold pressure no manifold pressure, rail is limited to 50-60 psi max, as the manifold pressure increases, a plunger is pushed over allowing more fuel to be delivered up to the pump spec maximum pressure, one of those catch 22 things, gotta have manifold pressure to get fuel pressure and vice versa. It is a delicate balance to get it right and if the bellows that the manifold pressure pushes against rips or gets a hole in it, the engine only gets the 50-60 psi no air fuel all the time, regardless of engine load or speed resulting in low/no power, all the time. On the pumps in the military truck, the AFC portion of the pump has a plug in it or it has an old non afc housing...the ones with the round area around the throttle shaft and 2 screws facing straight up, it ain't there, so, the pump works or it don't. Usually, the only thing that will cause an intermittent lack of fuel delivery issue is air or a restriction in the system or something rolling around on the floor getting under the go pedal and not letting it go to the full fuel position.