M931 Fuel prime problem

[D6c]

In addition to the air brake problem I posted earlier, I have a problem with the fuel system not holding prime.

Before starting the truck I have to bleed the air out of the fuel lines with the hand priming pump. Once I do that it will start right up and run fine.
If I shut it off for 10 minutes I have to bleed it all over again.

When I first got the truck someone had turned in the screw on the solenoid fuel shut-off so you couldn't shut the engine off without pulling the emergency fuel shut-off. I backed off the screw on the solenoid valve and now it works like it should.

I've heard the tank selector valve on the floor by the door can cause fuel priming problems, but this must be a pretty good leak to lose prime in just a few minutes.

Any idea what the problem is?

Thanks

 

[Csm Davis]

Have you changed all of the rubber fuel lines?

 

[D6c]

Haven't changed any lines or fixed anything other that what I listed above.....looking for a logical place to start.

 

[Csm Davis]

Start with the rubber lines even if they look good that will cure the problem 9 out of 10

 

[Hooper]

In addition to the air brake problem I posted earlier, I have a problem with the fuel system not holding prime.

Before starting the truck I have to bleed the air out of the fuel lines with the hand priming pump. Once I do that it will start right up and run fine.
If I shut it off for 10 minutes I have to bleed it all over again.

When I first got the truck someone had turned in the screw on the solenoid fuel shut-off so you couldn't shut the engine off without pulling the emergency fuel shut-off. I backed off the screw on the solenoid valve and now it works like it should.

I've heard the tank selector valve on the floor by the door can cause fuel priming problems, but this must be a pretty good leak to lose prime in just a few minutes.

Any idea what the problem is?

Thanks

I had a similar issue with mine. After going over all the fuel lines, removing the fuel tank, it turned out to be the small plastic line right at the manual primer. Taped it up, tightened it up and it was perfect.

 

[gottaluvit]

My line from the lift pump to the large fuel filter had small cracks and I went ahead and changed the one on the other side of the filter as well. Lots of folks said the square washer under the bolt holding that fuel filter canister in place gets warped and sucks in air there as well. The filter canister also has a gasket that could be leaking.
There is a thread on rebuilding the tank selector valve that is very informative and can be found in the search function. After all of this I still only ran on left tank until I noticed two new lines going to the selector valve that had been changed very recently but installed backward. Switched them and now runs flawlessly on both tanks.

 

[gottaluvit]

My line from the lift pump to the large fuel filter had small cracks and I went ahead and changed the one on the other side of the filter as well. Lots of folks said the square washer under the bolt holding that fuel filter canister in place gets warped and sucks in air there as well. The filter canister also has a gasket that could be leaking.
There is a thread http://www.steelsoldiers.com/showth...el-tank-selector-valve-cleaning-rebuild/page5 on rebuilding the tank selector valve that is very informative and can be found in the search function. After all of this I still only ran on left tank until I noticed two new lines going to the selector valve that had been changed very recently but installed backward. Switched them and now runs flawlessly on both tanks.

 

[D6c]

Thanks, I'll start checking the suggestions. It's likely something simple but they can sometimes be difficult to find.

When I got the truck there was a maintenance sheet that came in the packet. I remember it had a note about the fuel problem but there weren't any details.

 

[nk14zp]

Might be the o rings in the primer pump.

 

[D6c]

Upon inspection I found that both the supply line & return line at the fuel pump have cracks. Pulled the tank switch valve and 3 or 4 of the six lines there have cracks. There also was a metal return line that had the fitting nut pretty loose....possibly the main source of air leak.

While I've got the valve off I cleaned it up and replaced the o-rings. I found that a #111 & #124 o-rings fit the valve...that's different from what was listed in this previous thread. http://www.steelsoldiers.com/showthread.php?76086-5-ton-fuel-tank-selector-valve-cleaning-rebuild Not sure why the difference.

All but two of the short lines on the valve have been previously changed over to Push-Lock fittings. I can get additional fittings from http://www.discounthydraulichose.com

Decided to convert the fuel supply & return lines over to push-lock too. You can get "truck" hyd. hose but it was proving difficult to find crimp-on 90° flare fittings (most everything is 37° JIC) plus the hydraulic fittings are a lot bulkier than the originals and might cause clearance problems.

All the hose, fittings, & shipping will come to about $75, which isn't too bad.

 

[D6c]

Might be the o rings in the primer pump.

I had previously replaced the priming pump o-rings. They were kind of and odd size but found o-rings that would work.

 

[joesco]

Converted mine to a single tank, driver's side with a dual spin on filter setup. Works like a dream. I now use my passenger tank for off-road diesel storage for my diesel loader and genny.

 

[D6c]

Converted mine to a single tank, driver's side with a dual spin on filter setup. Works like a dream. I now use my passenger tank for off-road diesel storage for my diesel loader and genny.

Using the 2nd tank for off-road fuel sounds like a decent idea.....as long as you don't run into an overzealous DOT officer that tries to accuse you of running dyed fuel on the road.