1999 GMC 6.5L fuel problem

[mmlaw2002]

I have been getting air in my fuel system since I changed the fuel tank. It appears that the system becomes air bound. I have no fuel coming out of the top of the fuel filter even when the lift pump is pump. If I disconnect the fuel line (between the lift pump and fuel tank) the fuel starts to gush out of the fuel filter bleeder. This truck can run at idle for 2-3 hours without any symptoms....drive for 10 mins and the fuel system becomes air bound. I have checked and rechecked all connections....Please help!!!!!!!!

[handcannon]

Are you absolutely certain the LP is working all the time? Do you have a fuel pressure gauge you can moniter the pressure with?

For the past three weeks I have had air problems, only in the reverse of what you describe. I can run at speed as long as I want, but if I let it idle for very long it dies because air gets into the system. Using a fuel gauge I finally came up with a possible solution. The OPS (oil pressure sensor) operates two 12v circuits. One operates the oil pressure gauge in the dash. The other supplies 12v to operate the LP after oil pressure rises. In my situation the OPS works fine until the motor has warmed up, then the LP circuit quits supplying 12v to the LP (confirmed with gauge and added push button supplying 12v from another source to LP). The injection pump then takes over, sucking fuel up to the IP, creating a vacuum in the lines. This vacuum allows air to enter somewhere that would normally be seeing pressurized fuel from the LP. I think mine is coming in around the fuel manager as I have had problems with pressurized fuel leaking from it. I ordered a new OPS from RockAuto and just received it but haven't had time to install it yet. As soon as I can get it installed I'll be able to confirm my theory. Also, as soon as I can afford to I'm going to replace the FM with the RACOR 230 filter system that MGW has reccommended in the past as a good system.

Don

[JohnC]

... since I changed the fuel tank. ... If I disconnect the fuel line (between the lift pump and fuel tank) the fuel starts to gush out of the fuel filter bleeder. ...

Did you by chance put a new fuel cap on it, or parts from a gas truck? Sounds like vacuum in the tank, not air, is the problem...

[mmlaw2002]

I used the correct tank and fuel sending unit......with regard to the oil switch ..it has been replaced and when i run the engine the LP is wired direct to the battery to remove any of the issues you have raised.

[JohnC]

If I disconnect the fuel line (between the lift pump and fuel tank) the fuel starts to gush out of the fuel filter bleeder.

Still sounds like a vacuum in the tank.

[mmlaw2002]

I tend to agree but what could be causing this problem?????

[JohnC]

I tend to agree but what could be causing this problem?????

A lack of air...

Check the vent on the fill cap.

[Burning Oil]

Still sounds like a vacuum in the tank.

Or a pluged line or dirty sock. Try blowing it out from LP back to tank with cap removed.