Fuel System Troubleshooting and final resolution (IT WAS THE SOCK)

[98G]

I would guess the lift pump is OK, I have never seen one work at idle and fail at speed.
So it sounds like there is a restriction somewhere, could be the pickup sock in the tank, the check valves,, honestly that should be one of the first thing any HMMWV owner should do, I don't remove them from trucks I sell because we live in a litigious society.
How did the filter and housing look???
[/QUOTOK?

I had an M936A2 where the lift pump worked fine at idle and even revving with no load. It even passed the TM's trouble shooting tests. But once i ruled everything else out and started throwing parts at it in desperation the lift pump was the first thing I changed, and that resolved it.

Almost identical symptoms.

If the check valves are OK, I'm ordering a lift pump... it's $30.

 

[Mogman]

Filter and housing were terrible. Makes me suspicious of the fuel tank. Filter is now new and housing was thoroughly cleaned.

I got derailed and didn't get the check valves today. It's on the agenda tomorrow.

I checked the lines and didn't see any kinks or crimps.

Doesn't the fuel coming out of the water drain pretty much demonstrate that the lines and pump are ok?

There is one other possibility, since the fuel filter was in bad condition (how long has it been since you changed it?)

The lift pump is going to try and push that fuel through the filter and if it is clogged It is possible it pushed crap past the filter and that the filter screen on the IP fuel inlet is now clogged, it is a real PITA to get to, even harder on the NA IIRC.

This would of course be the last item after you have eliminated any and all other possibility's

 

[98G]

Ok, looking at the lines coming out of the tank. I've got two soft lines between the hard lines coming out of the tank and the hard lines going forward. This is where I'd expect the check valves to be.

Looks to me like someone already deleted them. Am i correct?

The larger one shows some cracks. That's the supply side. On a 5ton, such cracks would be suspicious for sucking air and not running for **** . I'll be putting some new rubber line here tomorrow. Does anyone think this is the culprit lesion?

I don't know HMMWVs, obviously.

Pic is lying under the truck looking up at the fuel tank.

 

[98G]

So I replaced the soft lines and started the truck.

Starts and runs fine, just like it has been doing. This time I expected it to crap out after running for a few minutes, so I drove it in circles in the field.

Sure enough, after just a couple of minutes of acting normal, it acted starved for fuel and died. I opened the driver side drain with the truck off, and nothing came out. I cranked with the drain open and got a sluggish stream. The truck started and loped. The stream got stronger, then got weaker and the truck died.

If this were a heart patient, I'd say it acts like cardiac tamponade. @TOBASH

I'm thinking the diaphragm in the lift pump tore and fuel is getting past it. When enough fuel gets past it, the negative space becomes full of incompressible fuel and it loses the ability to expand and pump. That accounts for the delayed onset.

I'm going to order a fuel pump this evening, unless anyone has a better idea.

 

[TOBASH]

So I replaced the soft lines and started the truck.

Starts and runs fine, just like it has been doing. This time I expected it to crap out after running for a few minutes, so I drove it in circles in the field.

Sure enough, after just a couple of minutes of acting normal, it acted starved for fuel and died. I opened the driver side drain with the truck off, and nothing came out. I cranked with the drain open and got a sluggish stream. The truck started and loped. The stream got stronger, then got weaker and the truck died.

If this were a heart patient, I'd say it acts like cardiac tamponade. @TOBASH

I'm thinking the diaphragm in the lift pump tore and fuel is getting past it. When enough fuel gets past it, the negative space becomes full of incompressible fuel and it loses the ability to expand and pump. That accounts for the delayed onset.

I'm going to order a fuel pump this evening, unless anyone has a better idea.

If this is truly a lifter problem, you need to replace it immediately before Diesel fuel gets beyond the diaphragm and thins out the oil and you score your cylinders and bearings or even seize the engine.

 

[TNDRIVER]

So I replaced the soft lines and started the truck.

Starts and runs fine, just like it has been doing. This time I expected it to crap out after running for a few minutes, so I drove it in circles in the field.

Sure enough, after just a couple of minutes of acting normal, it acted starved for fuel and died. I opened the driver side drain with the truck off, and nothing came out. I cranked with the drain open and got a sluggish stream. The truck started and loped. The stream got stronger, then got weaker and the truck died.

If this were a heart patient, I'd say it acts like cardiac tamponade. @TOBASH

I'm thinking the diaphragm in the lift pump tore and fuel is getting past it. When enough fuel gets past it, the negative space becomes full of incompressible fuel and it loses the ability to expand and pump. That accounts for the delayed onset.

I'm going to order a fuel pump this evening, unless anyone has a better idea.

Pull the vent hose off the top of the mech. pump and crank/run the engine, your answer is in what does or doesn't squirt out the vent tube on the top of the pump.

 

[98G]

If this is truly a lifter problem, you need to replace it immediately before Diesel fuel gets beyond the diaphragm and thins out the oil and you score your cylinders and bearings or even seize the engine.

Thanks. Checking the oil for fuel should give me a definitive diagnosis then.

 

[TOBASH]

No definitive. Diaphragm and unit can fail many different ways

 

[98G]

Pull the vent hose off the top of the mech. pump and crank/run the engine, your answer is in what does or doesn't squirt out the vent tube on the top of the pump.

Nothing comes out. It took some cranking (as if it had lost prime) to get it running, then it loped for a few seconds and then smoothed out and ran normally with the vent hose removed. I set the high idle and let it run at a guessed 2000rpm for a while. I disengaged the high idle. Then I closed the hood and drove it around in the field.

After about the same amount of time, it acted starved for fuel and died.

Crankcase oil shows no sign of fuel contamination. It isn't overfull, and does not smell like diesel.

I'm still thinking it's the lift pump, mostly because I'm at a loss for what else it could be.

 

[marchplumber]

Nothing comes out. It took some cranking (as if it had lost prime) to get it running, then it loped for a few seconds and then smoothed out and ran normally with the vent hose removed. I set the high idle and let it run at a guessed 2000rpm for a while. I disengaged the high idle. Then I closed the hood and drove it around in the field.

After about the same amount of time, it acted starved for fuel and died.

Crankcase oil shows no sign of fuel contamination. It isn't overfull, and does not smell like diesel.

I'm still thinking it's the lift pump, mostly because I'm at a loss for what else it could be.

When my lift pump leaked on my M1009.....i had diesel all over ground.....i didnt have your symptoms, but different trucks.....mine was a bugger to change, hard line was hard to get started, tight space on M1009.....It wasnt expensive, wasnt terrible to do....id change it out if I was you, new one aint a terrible thing anyhow, fix or no fix.....2 cents

 

[98G]

When my lift pump leaked on my M1009.....i had diesel all over ground.....i didnt have your symptoms, but different trucks.....mine was a bugger to change, hard line was hard to get started, tight space on M1009.....It wasnt expensive, wasnt terrible to do....id change it out if I was you, new one aint a terrible thing anyhow, fix or no fix.....2 cents

That's where I'm at. $40 and it isn't like a new lift pump is a bad thing. It doesn't look hard to reach.

I generally dislike diagnosis by throwing parts at it, but I've looked at every thing else.

I'll update this when the new one is on.

Thanks everyone who contributed.

 

[Mogman]

Be very careful that you have the fuel pump rod fully seated in the block before installing the new pump or you will break the pump rod.
I pull the small plate that is behind the pump held on with 2, 1/4" screws, then grease the rod with thick axle grease and re-install the rod, the plate and then the pump.
I start the pump bolts through the plate then tighten the 1/4" screws, then remove the pump bolts and install the pump, this insures the threaded holes in the block are aligned with the small plate, the gasket for the plate is the same as any early (327,350 etc) chevy small block engine.
Some just put grease on the rod and push it back in without pulling the plate, as long as you get the rod to stay in place you are good to go,, I feel being able to completely cover the rod including the end that goes against the camshaft helps make sure it stays in place and of course replacing the gasket helps to insure against leaks.

 

[Mogman]

This is basically what you will be faced with, despite being a gasser the design is exactly the same.

 

[Mogman]

This is basically what you will be faced with, despite being a gasser the design is exactly the same.
View attachment 932247

Actually not exactly the same, if you look you can see a 3/8" bolt in the block behind the harmonic balancer, that threaded hole in the block intersects the push rod bore, on all the GM gassers you simply remove that short bolt and install a longer one and carefully snug it against the pump rod whilst holding it up in place then remove that bolt and re-install the short one after the new pump is mounted.

Evidently the dingleberries that designed the Detroit forgot this simple solution.

 

[98G]

Actually not exactly the same, if you look you can see a 3/8" bolt in the block behind the harmonic balancer, that threaded hole in the block intersects the push rod bore, on all the GM gassers you simply remove that short bolt and install a longer one and carefully snug it against the pump rod whilst holding it up in place then remove that bolt and re-install the short one after the new pump is mounted.

Evidently the dingleberries that designed the Detroit forgot this simple solution.

I've done a 350 Chevy, but it's been like 40 years ago. I intend to watch a video on this one before I attempt it.

I hope not to do anything stupid. Your input helps in this.

I'd like to not remove that second plate if I can help it. I'm thinking maybe a flexible piece of plastic to hold the pump rod up while getting the pump lever in position. I saw a suggestion for cutting a 3/4" x 7" strip out of a plastic container to use for this.

 

[TNDRIVER]

I've done a 350 Chevy, but it's been like 40 years ago. I intend to watch a video on this one before I attempt it.

I hope not to do anything stupid. Your input helps in this.

I'd like to not remove that second plate if I can help it. I'm thinking maybe a flexible piece of plastic to hold the pump rod up while getting the pump lever in position. I saw a suggestion for cutting a 3/4" x 7" strip out of a plastic container to use for this.

My guess now........ messy fuel tank, not a pump. The "butter fly" valves can break in them but do not have the symptoms you describe. Less trouble to pull the fuel sender and eyeball the tank than replace a pump. Just my 2 cents. Please post a picture of the inside of your tank.

 

[DREDnot]

Im voting for lift pump. Ive had numerous lift pumps leaking externally at the seam upon arrival from gov planet. Ive also had trucks that had the same symptoms you described with good flow at idle but would starve for fuel while driving. Good flow at drain valve while idling but weak flow following starvation. Pump necropsies always revealed cracked/deteriorated diaphragms. Ill bet yours has a few small cracks or rubber loss from the diaphragm.

 

[TNDRIVER]

Im voting for lift pump. Ive had numerous lift pumps leaking externally at the seam upon arrival from gov planet. Ive also had trucks that had the same symptoms you described with good flow at idle but would starve for fuel while driving. Good flow at drain valve while idling but weak flow following starvation. Pump necropsies always revealed cracked/deteriorated diaphragms. Ill bet yours has a few small cracks or rubber loss from the diaphragm.

Maybe.......... had one leaking from the edge as you describe, but ran OK. Had the fuel tank off one TWICE and the second truck once. Both were nasty. I'm holding my breath!

 

[Mogman]

I've done a 350 Chevy, but it's been like 40 years ago. I intend to watch a video on this one before I attempt it.

I hope not to do anything stupid. Your input helps in this.

I'd like to not remove that second plate if I can help it. I'm thinking maybe a flexible piece of plastic to hold the pump rod up while getting the pump lever in position. I saw a suggestion for cutting a 3/4" x 7" strip out of a plastic container to use for this.

I am just not sure why so many resist removing that plate, it is only two small screws and then you do not have to stand on your head, hold your tongue just right and worry about getting the rod out of the way, seems like a no brainer every time I do it, but whatever gets the job done

 

[Milcommoguy]

Im voting for lift pump. Ive had numerous lift pumps leaking externally at the seam upon arrival from gov planet. Ive also had trucks that had the same symptoms you described with good flow at idle but would starve for fuel while driving. Good flow at drain valve while idling but weak flow following starvation. Pump necropsies always revealed cracked/deteriorated diaphragms. Ill bet yours has a few small cracks or rubber loss from the diaphragm.

Good luck on your fuel problem. Working one here too. On a slightly different note, another GOOD LUCK finding a quality USA pump. Even AC Delco made in Korea. One had a cheap ass crimp ring that came off the lever arm end... and that ruined the day. Lasted about 3 month. Good on the outside ...Crap workmanship on the inside. Where are all the good parts? Willing to pay once.

New language class at the kids HS. Mandarin 101. Can't make this poop up.

Waking up in America, CAMO