air in system

[gilbertmonchat]

hello, i have air in my fuel filter , and my sub run roughly antil he die, i change fuel line from lift pump to filter , filter to IP and filter to drain valve, change filter too, but always the same problem , when a bleed the air by filter cap the truck run smooth for 5 to 10 minutes and after the air coming back to fuel filter, i check the line after the lift pump to tank but nothing wet, and my tank is full, did i have to change the return line too, if somebody have a test to find some leak (air) on system !!!!

[DmaxMaverick]

You likely have an air leak between your lift pump and tank pick-up tube. Probably the fuel level sender unit (float) or the supply line attached at the top of the tank. Return lines, or lines between the lift pump and injection pump will leak externally (makes a mess). Lines between the tank and lift pump will suck air. All this assuming your lift pump is working properly. If the lift pump is not working, it will be suction from the tank to the injection pump. If you were able to bleed the air via the vent at the top of the filter, the lift pump is working. Are you able to get fuel pressure with the engine running (open the bleeder and it runs out)? If so, your lift pump is working all the way around.

[gilbertmonchat]

i have pressure at the cap filter unit the fuel come at 2inch high over the cap, but i find a really small leak fuel at the return line, for the rest of the line between the tank and the lift pump, can't see anything.

[DmaxMaverick]

Sounds like your lift pump is fine. The return leak can make a mess, but should not cause running issues. You very likely have a leak between the lift pump and fuel tank. That is the only way to have air sucked into the system with a working lift pump. Check the fitting that connects to the lift pump supply line (in), and check the condition of the line from there to the tank.

You can supply low air pressure to the tank to find the leak (no more than about 5 PSI). You can install a tire valve stem into a fuel cap to supply the air. With air pressure on the tank, it will leak fuel if the leak is outside the tank. If no leak, it is probably the fuel sender tube.

[Mark Rinker]

I just got through a similar rough running episode with air at the filter. Agree that you should investigate the tank pickup tube and fuel lines running to lift pump.

Mine was rusty, but dry. We replaced a section of fuel line last spring and it got better, then started acting up again. Replaced sender this winter and it is running well again.