Losing Prime

[sandcobra164]

I've got a CUCV at work that is losing it's prime when it sits overnight. The shop guys have tried replacing the fuel filter, tightening some clamps on fuel lines but the problem is still there. The truck is another one of those Norway Cave trucks with less than 4,000 miles on it. My personal M1028 and one other at work do not have these issues but #3 does. What are the common places to check on a CUCV for air to infiltrate the fuel lines. As a side note, once primed, the trucks gives no trouble what so ever running, shutting off and restarting within an hour or two, and will start right up in the morning till it hits the air bubble. Then it starts with extended cranking. Glow plugs are good and no fuel leaks that we can find.

 

[Matt65]

Factory fuel cartridge filter, IP return, Rubber fuel line in engine valley.


http://www.steelsoldiers.com/cucv/65334-truck-shutting-off-after-driving-return-valve.html

 

[Dr.Jay]

Leaking rubber hose at the fuel sending unit.

 

[tstone]

My guess is if it is leaking between the lifter pump and the IP you would see obvious fuel leakage or seeping due to the lifter pump pressure. If it is a leaking diaphram in the lifter pump you may smell it in the crankcase oil or see a slight diference in the oil level overtime. If it is between the tank and the lifter pump you should check the rubber hose conections at the tank and the lifter pump. Don't disreguard the steel line, it may be fatigued or damaged enough to leak overnight. Lastly test seperate sections of your lines (with components conected) with a vacuum or pressure gauge to test for leaks. do it in 2 or three sections until you isolate it. Pressure test:5psi, vaccuum, 5 in.

 

[Hasdrubal]

The number one cause of CUCV's losing prime has been the fuel pressure sending unit located in the back of the fuel filter base. You dont need it, its for the military STE/ICE testing equipment.You can replace it with an OEM unit that doesn't have the sensor, replace the o-ring or tap and plug the orifice, some say you can JB weld it. As a temp fix, crack the fuel cap after stopping at night or do what I did. It took a while to get a new filter base so I drilled a small hole in the cap so I wouldn't forget to open it one night.

 

[lavarok]

I'll vote lift pump or filter base. Start with the filter base and go from there!

 

[jeffhuey1n]

I replaced all the above and it came down to the fuelfilter. There is a very thin O ring about the size of a 50 cent piece. It is cracked or broken. Replace the O ring OR get rid of the whole box filter set up and get the screw in type NAPA filter. I replaced the O ring and it still lost it's prime (the minute cracks or breaks in the line aren't enough to dump the prime right away; let it set over night and you have to reprime to whole system. I lucked out and found a custom commercial two part filter system on a scrapped suburban. Since the holes matched up perfectly, I just used the secondary mount and bolted it to the bulkhead using the factory holes. See attached picture). There's also a thread on building a scrath built filter system, not sure where but I used the searchfeature and it popped right up.

 

[patracy]

When it comes to diesels, just start replacing any rubber you can.

Check all the metal lines from the lift pump to the fuel filter housing as well.

The lift pump works as a check valve as well. Save it's replacement for last. Check the rubber hoses at the tank sending unit as well.

I had this same issue when I installed the turbo setup on my CUCV. Turned out the past owner of the "kit" used some fuel hardlines on the electric fuel pump. He used some o-rings to seal them. One had a slight tear in it. And caused the truck to run fine for a few seconds, then lope and die. After purging air three times, I knew I had a air leak.