M939A2 Adjustable return fuel check valve.

[idbankrman]

The part number for the lift pump I bought from Highway and Heavy parts is 4988749. It looks like the one you see on most trucks with the fuel line connection on top and the hand primer on the bottom. It is not the original that the trucks came with but an improved replacement. You won't have to make any modifications to use it. The military uses this one as an exact replacement just as often as the original style pump. Oh the pump was actually $43 + 13 shipping

Thanks for all the input on the priming issue. I pulled the rubber line from the canister filter to the lift pump and it looked very shabby with lots of cracks. So I changed it and immediately the truck was much easier to re-prime and start. I'm going to give it 48 hours and then see what happens. I had NAPA make a replacement using hydraulic hose. If this isn't the final answer then Im going to pull the tank selector valve (this is M929A2 dump truck) and rebuild it and replace the small rubber hoses. I would go right for an inspection of the T fittings at the fuel tanks but on these dump trucks the fuel tanks are covered by a metal plate and the spare carrier so it's a big job doing any fuel tank work.

 

[Plane Fast]

Thanks Idbankrman.

 

[Csm Davis]

The part number for the lift pump I bought from Highway and Heavy parts is 4988749. It looks like the one you see on most trucks with the fuel line connection on top and the hand primer on the bottom. It is not the original that the trucks came with but an improved replacement. You won't have to make any modifications to use it. The military uses this one as an exact replacement just as often as the original style pump. Oh the pump was actually $43 + 13 shipping

Thanks for all the input on the priming issue. I pulled the rubber line from the canister filter to the lift pump and it looked very shabby with lots of cracks. So I changed it and immediately the truck was much easier to re-prime and start. I'm going to give it 48 hours and then see what happens. I had NAPA make a replacement using hydraulic hose. If this isn't the final answer then Im going to pull the tank selector valve (this is M929A2 dump truck) and rebuild it and replace the small rubber hoses. I would go right for an inspection of the T fittings at the fuel tanks but on these dump trucks the fuel tanks are covered by a metal plate and the spare carrier so it's a big job doing any fuel tank work.

Before you pull you selector valve try changing the rubber hoses that go to it, not saying that they don't go bad but I have pressure primed hundreds of the tractors and wreckers and have only seen one leak on the selector valve 90% were the lines in and out of the valve and on A2 models for some reason the lines going in and out of the fuel filter were more likely to be leaking.

 

[Vintage iron]

Watching this thread. I have 8.3L that is running weak and loosing prime when it is shut off. The truck will run weak, then I shut it off it loses prime. I have to power bleed the injection pump all the way to the injection lines to get the prime back.

 

[Hard Head]

Had the same issue out of the blue with my 923A2. Mine was a slightly different issue as the elbow had cracked on the IP. I had to reverse drill it out as it broke off when I removed the fuel line. I decided to follow the info on this thread and change out the original fuel / water filter with a spin on. My original filter base had a schrader valve to pressurize the system. Since that went away, I added a priming pump (facet 24v) in parallel between the IP and new filter. I had to put a check valve between the main IP and filter line to prevent back flow when running the priming pump. I am going to replace the hose from the original filter to the tank once NAPA gets some more in tomorrow. Going to cut some steel plate to mount the new filter, pump, and check valve. I also have to put the priming switch in the dash. Once I finish I will post the pics. So far it primed with no issues. I didn't like the manual priming pump! Have to finish up before cold weather

 

[idbankrman]

Cam Davis Thanks for the tip on rubber lines on the selector valve. I've looked them over and they are showing small surface cracks. I'm awaiting better weather to mess with them.

 

[Csm Davis]

Cam Davis Thanks for the tip on rubber lines on the selector valve. I've looked them over and they are showing small surface cracks. I'm awaiting better weather to mess with them.

Any fuel lines I have seen with cracking was leaking or about to start. I like the push-lok type fittings and hose for repair of fuel lines because all you really need to repair them is a knife to cut off the old hose and a new piece of hose to push on in it's place. Not as easy as it sounds but not terribly hard.

 

[86m1028]

I know this is an older thread but wanted to post. I searched & never came up with this thread.
I posted in another older thread about fuel prime issues & MtnSnow mentioned the check valve in the return line.

Truck in question is an m936A2 (wrecker, 8.3)
At auction site, infamous "ran when parked"
Should have known there was an issue when I get there, both tanks are FULL & I mean full to the top, shrader valve in fuel tank cap.
I pressurized fuel tank to prime truck, ran rough, not enough power to drive on trailer.
Truck was trailered about 55 miles, would not restart, rolled truck off trailer.
Truck would not prime from either tank, to move truck where I needed it, I rigged up a 5 gallon can inside engine compartment.
1st replaced both rubber lines, (metal line to large fuel filter & fuel filter to prime pump) no change
2nd replaced all 6 rubber lines at selector valve & rebuilt selector valve no change
3rd replaced both (large frame & small engine) fuel filters no change
4th checked fuel tank pickup tube (had to remove tank because wrecker bed blocks all access to top of tank no issues
5th replaced complete line from tank to prime pump (by passed fuel selector valve & large frame filter) no change
6th ordered Tork tech adjustable fuel return check valve & will post back.

 

[quarkz]

Are you looking for fuel at the injector, by loosening a fitting, or are you saying each step was a failure because the engine didn't start? I would look for fuel at different points along the path. Also check the fuel cut out switch and mechanism on top of the IP. Is the 936 a Cummins nhc250?

Sent from my Galaxy Nexus using Tapatalk

 

[86m1028]

Are you looking for fuel at the injector, by loosening a fitting, or are you saying each step was a failure because the engine didn't start? I would look for fuel at different points along the path. Also check the fuel cut out switch and mechanism on top of the IP. Is the 936 a Cummins nhc250?
Sent from my Galaxy Nexus using Tapatalk

Each step was a failure after priming the truck, getting it started, shut it down & after 15-20 minutes, trying to restart the truck.
I did try checking for fuel at different points.
The fuel shutoff solenoid is working correctly.
The 936 is the wrecker version, it's an A2 which means it's the 8.3.

Truck is loosing prime at the IP, it seems to be draining back into the tank either thru the return line or the supply line.

 

[kmfltd]

Very old thread but does anyone make adapter fittings to change these fuel line fittings from what they are (no idea what they're called) to straight AN fittings that can be sourced anywhere, I just don't like push lock and I want to go to braided steel lines instead of rubber.

 

[WillWagner]

Very old thread but does anyone make adapter fittings to change these fuel line fittings from what they are (no idea what they're called) to straight AN fittings that can be sourced anywhere, I just don't like push lock and I want to go to braided steel lines instead of rubber.

For the return? You can use a #4 an, but, no need to go high dollar on a return line.