I have a 1986 F150 302 EFI with dual tanks. It will not start. The problem started after I replaced the water pump--I posted with this problem before. Well I've eliminated, as far as I can tell, broken wires in the ignition system from moving stuff around when I replaced the water pump.

When I bought this truck, it would not run when I selected the rear tank. The front worked fine so I just put off doing anything about it. A few times in a year and a half I have had to siphon gas off the rear tank because it was overflowing.

The engine turns over fine (good battery) and there is fire to the spark plugs. Both tanks have plenty of gas. I tested the fuel pumps by grounding the test connector. The high pressure pump runs. I couldn't hear the pump in the front tank; I didn't know how loud it should be. Well just to check everything, I switched over to the rear tank. I could clearly hear that tank's pump. The rear tank is the one that never worked. I tried starting it with the rear tank and it ran rough for about 10 seconds and died and wouldn't restart. This is how it has always behaved when trying to use the rear tank.

I just always assumed the pump in the rear tank was bad, but I never did this test before. When I first tried starting it after the water pump replacement it ran about 20 seconds and died and wouldn't restart.

Does this sound like it could be the tank selector valve? My Hayne's book says to apply 3-5 psi air pressure to test it but I don't have anything to do that with. I checked the fuse and it's OK. It does seem to be switching between the two tanks, so I don't think it's the wiring or dash switch.

Sorry for the long post. Any ideas would be greatly appreciated.

Rick

 

Sounds to me like the switching valve in your truck is bad. A quick way to test this is to just bypass the switching unit under the truck and see if it will start. If it does you have your problem.

 

Pull the fuel line off and put it into a bucket somehow. Then do your fuel pump testing. This will confirm if you are on the right track, since it sounds like you have a good idea it's a fuel problem, but still don't sound very sure.

 

Thanks, I'll try testing the pumps with the lines disconnected.

Rick

 

I second the diagnosis about the problem with the valve, or the hoses are misrouted.

The way these valves work on your truck is the following...

Fuel is pumped to the engine via the intank fuel pump from the tank you have selected, through the valve to the engine. All fuel that is not used is returned to the tank it came from. If the selector valve is bad, or the hoses misrouted, the fuel system can literally empty one tank and fill the other which is what it sounds like is happening.