77 F100, 300 6 cyl 2WD shortbed. non electric fuel pump.

Refurbished the fuel system (new tank, tank sending unit, rubber fuel hoses, filler hose, tank vent hose) did some short drives and it ran fine, but now it is losing the prime in the fuel line. It will run for a couple of minutes then die. If I feed about a pint or so of fuel into the fuel line (behind the pump) it will start and run. It is also very reluctant to take fuel, only a splash at a time then backs up and overflows. The tank only has about 3-4 gallons in it and show 1/3 tank. It also has a screw on filler cap similar to the 87-92 models. I say that because my 86 has the bayonet style filler cap. The stock filler and vent hoses installed without a problem.

Seems to me a pressure problem with the vent system. Tank isn't venting properly, creates a suction, pump sucks up the fuel in the lines then engine quits till it is re primed. I'm thinking maybe the vent is clogged in the filler neck and or a bad (non venting fuel cap)fuel cap.

 

Try running her with the gas cap very lose or off. If she runs, you know the problem is in the venting system, like you think. If the problem still exists we'll look elsewhere - blow out the lines to the pump, after the pump, or try something completely different.

 

Quick update:

It's been too hot here to do much outside. Still recovering from some surgery. Worked in the garage with A/C on a BMW.

Today after evicting the wasps I found out the tank vent tube was probably not routed correctly. It has a filler neck from a later model and has been rigged up to work. The vent hose ran under the filler hose so I suspect it filled with fuel on fill up and would not vent. I rerouted it to run above the filler hose so I hope that works. Also plan on a new gas cap.

As for the fuel lines I disconnected the hose from the hard line near the tank and ran some hose from the hard line into a bottle. I then disconnected the hose aft of the fuel pump and blew it out with compressed air. I got about four ounces of old gas and some grit. As I suspected the only fuel I was burning was the fuel I used to prime the line. I'm getting good air flow thru the lines so I think maybe the fuel pump is weak. I'll replace the fuel pump. prime the lines and go from there.