939A2 keeps losing prime, no leaks evident

My 923A2 keeps losing prime to the injection pump over the course of about a day. Injectors appear to remain primed or mostly so. It seems to be prime up to the IP that is being lost.

So I replaced the spin on fuel filter the other day, hoping that would fix the problem. It didn't.

This truck does not have the canister filter as some do. And there are no leaks evident anywhere in the system. Not at the fuel pressure transducer nor at the injector pump, or the vent on the injector pump nor any of the lines to/from the fuel tank.

I am at a loss. The only thing left I can think of might be the fuel pickup tube at the tank? But I'm not really sure how to go about checking that out.

Not a major issue, but it is annoying and I'm sure it's not good for the system as I do live in a pretty humid area.

Pressure the fuel tank and then check to see if any of the rubber lines have pinhole leaks. I had the same problem and I found that I had to replace a fuel line.

How did you pressurize your fuel tank?

Pressure the tank by drilling a hole in the Fill Cap and add a Wheel Valve Stem (schrader valve) or take the Vent tube from the top of the tank and blow air from there... I would bet the line from the frame rail to the fuel pump is dry rotted.
Other common problems are tank switch valves on Tractors (M931 M932) and Pickup tubes in the tank being cracked or broken at the fitting inside the tank on top...

Hope that helps

Do not over-pressurize the fuel tank. 5 PSI or so will be enough. You may have to do it several times to see a leak.. It may only be a weep.

Hey Texandy I have a 925A1 and had the same issue as your having, it was the main fuel line running INTO the IP, it was collapsing inside of the line, I replaced that fuel inlet line and never had another problem. A military mechanic friend told me that at time troops would run JP-8 in these trucks and apparently that stuff is very hard on the fuel systems in these trucks.
Hope this helps you
KK

Karl kostman said:

Hey Texandy I have a 925A1 and had the same issue as your having, it was the main fuel line running INTO the IP, it was collapsing inside of the line, I replaced that fuel inlet line and never had another problem. A military mechanic friend told me that at time troops would run JP-8 in these trucks and apparently that stuff is very hard on the fuel systems in these trucks.
Hope this helps you
KK

Click to expand...

JP-8 is all we used in the trucks. Unless you were doing a red ball mission and having to fill up at commercial stations, JP-8 was all you used on post for military vehicles.

Andy, if you don't have a modified fuel cap, as a field expedient, you can stick the end of your tire filler into the mouth of the filler neck, and "seal" the neck with a large rag, then give it some air. This will actually get enough pressure in the tank to force fuel through the lines.

One of my M931's had the same issue, no visible leaks, save one. After replacing the visible leak line, I still had problems. I replaced all the rest of my fuel lines, solved the problem (until my tank-switch valve started leaking.. grrr...)

Cheers

That's a good idea, but that actually gives me another idea that would make it a little more hands-free and secure...

I'll just take a PVC end cap and turn/thread the inside to match the tank cap threads on my lathe, then install a schrader valve on the pvc endcap. Should be safe enough at only 5 PSI.

I wasn't real thrilled with the idea of modifying my fuel tank cap.

That ought to work. The rag is for when you are stuck on the side of the road and goodies are few. Very little pressure involved, the tank usually makes a "bonk" noise when its had enough.

Cheers

I've having the same issue and I found that the primer pump is sucking air. No fuel leaks otherwise but when you pump it, it leaks a little fuel. So that's what I'm thinking is happening.

My 923a2 is also losing prime, that's how I got it from GL and thankfully Jeff at Sparta alerted me. He told me he had to pump the manual prime to start it and that I would have to replace the part (not sure what it's called, but it's on driver's side of motor and has a plastic round button that you depress about 1/2 inch repeatedly to re-establish prime. Anyway, I now treat this as a poor man's antitheft system. It is a PITA to open the hood and prime b/c it takes 100 pushes (I didn't count, but seems like it) of the button to start the truck. I am curious if it's actually the fuel lines or the prime part that Jeff mentioned. Maybe both are bad and the prime part doesn't overcome the entry of air from holes in fuel lines.

When I picked up my M936A2 at the GL yard at Ft Carson it would prime with the push button on the lift pump fine, start right up and run good for a few minutes then die. It was obvious it was sucking air from somewhere. Did this many time and the truck was towed off base for further investigation. We bought a inline fuel pump and inserted in the rubber fuel line coming from the hard line on the frame rail to the lift pump/primer pump on the block. The fuel filter shown in one of the picture was originally inserted in that line possibly by the unit. All the A2 series trucks sold at Ft Carson had that mod done to them. Anyone else out there have a fuel filter in the line from the hard line from the frame rail to thew lift pump? Makes sense to have a filter in there since without it the fuel is delivered unfiltered to the lift pump. Anyways back to the discovery. Once the the fuel line was pressurized with the new inline pump it was apparent that rubber line was bad. Fuel seeped through the cloth covering fairly quickly. Left the truck at a friends yard and had a new fuel line made up using the old fittings. That is the new fuel line in the pictures. No inline fuel filter. Replaced the line, primed it one more time (loosened 10mm bolt on injection pump) and it started right up and has been running perfect now for 3 months. It wasn't apparent that line wasn't bad until it was pressurized. It looked new from the outside so you just never know. I'm sure one if not more of the many soft lines on my M818 are in this same condition as evidenced on how it is to start after sitting for a few days.