air in fuel system

[camo85]

So I borrow my old truck I sold [85 camo in sig], cause the 97 is giving me problems.... and what happens.... that one decides it ran good for long enough. The fuel pump went out Friday [atleast I'm pretty certain it did], so I replaced, bled the air out, and we are off again.... right? So I try to drive to work Monday, and it's fine till the way home [hour drive each way], then it starts acting like it's runnin out of fuel. So I get home and crack the injectors and bleed bleed bleed.... test drive shows improvement. So Tuesday I leave for work and I only make it twenty minutes from home and there I sit. It keeps getting air back into the injection system somehow, to the point you would think the mechanical pump failed again. Is it possible this could just be return lines or bad injector O-rings? Or maybe a rotted pickup tube in both tanks? I see no physical signs of a fuel leak anywhere. But there is a significant amount of air getting in somewhere, so much that it is all that comes out of the shrader valve when you try to bleed at the filter. Any suggestions?

 

[Full Monte]

Remember....your fuel system is under vacuum between the tank and the mechanical fuel pump. Air can leak into the system anywhere there. Is there one of those old water separators on the firewall? If so, its likely to be coming from there. Bypass it and the problem should go away. Also check for air leaks all the way back to the tanks.

 

[camo85]

The old separator has long been bypassed! I will have to check a little more in detail closer to the tanks, maybe a rubber line I can't see with the tank in place? It just boggles me that it's doing it with both tanks, you would think it would just be one or the other, not both all of a sudden at the same time.

 

[icanfixall]

What level are the tanks... When was the fuel filter last changed. Check these items and post back.... If the tanks are near 1/4 tank you probably are running out of fuel because the pickups are broken off in the tanks... A real problem in most of the diesel trucks....

 

[camo85]

What level are the tanks... When was the fuel filter last changed. Check these items and post back.... If the tanks are near 1/4 tank you probably are running out of fuel because the pickups are broken off in the tanks... A real problem in most of the diesel trucks....

Just replaced the filter Friday after I did the mechanical pump. Tanks are hard to judge do to faulty sending units that have never worked right since I owned the truck, but I know there is about half in the one and probably 1/4 or so in the other. Is there a way to check the pickups without pulling the tanks down?

 

[icanfixall]

Yes.... The front tank can easily be checked. Just get under the truck and find the tank sending unit. Its held in the tank with a large spanner nut. A hammer and punch will turn it. Remove the lines and pull it out of the tank.... Tha rear tank needs to be dropped and thats a job and a half. The fuel thats left in it will move around causing the balance to shift. Kinda like trying to hold something really greasey in your hands... Also the likes to the rear tank needs to be cut loose from the cross member so you have more room to drop the tank and reach the lines. Like I said.. Its a tuff job for one person.

 

[camo85]

ok I'll have to check it out. I have droped fuel tanks before, just not in one of these trucks. I didn't think you could pull a sending unit out with the tank in place! I will try the front one then first before I go any farther to dropping the rear to see if either is broke off. Ijust can't fathem that both would break at the same time, there must be a hole somewhere else, but I can not find any fuel leaking anywhere. I'm hoping to just find a rotted rubber line near the tank, but haven't.

 

[camo85]

Got the problem fixed! My father-in-law [the new owner of the truck], came up and ran a new line from the tank switch valve to the lift pump. We also found a flaw on the flaired end [pressure side] coming off the lift pump and re-flaired it. Whichever of the two it was, it fixed it!

 

[dyoung14]

I was about to sugest the tank selector valve, because i had a similar problem about a week ago, and thats what it turned out to be, glad to here you got it fixed tho!