86, F150, 5.0, dual tank fuel system problem

[c150L]

I have been struggling with this for quite sometime now. What would help me most is a wiring diagram and/or fuel system component lay out. This 4x4 has the 5.0 EFI, has dual tanks and I don't seem to get fuel from either tank. The fuse for the fuel selector circuit had been pulled by someone, no idea why or when, but wonder if they took anything else out. The front (high pressure) fuel pump in the frame cycles normally, but I don't seem to have any action from the low pressure pump in either tank. I also am not reading any fuel in either tank. One tank has at least 5 gal, other closer to 9 gal. I can get the engine to start for short durations once in a while, but nothing sustained.

Someone please help!. I need this transportation despirately due to the death of our Explorer 2 weeks ago.

[nighthog]

I believe that there is a fuel tank selector valve located on the frame under the drivers door. If the motor was shorted out this could be why the fuse had been removed. If the valve is blocked or partially blocked this could cause your problem. The wiring curcuit for the fuel tank sending units also go through the selector valve which would explain the fuel gauge problem. Finding wiring diagrams is a real pain. I signed up with alldata for my 87 and was able to get some diagrams, but they were not 100% accurate. The curcuits were close, but connecter ids were not correct. I was able to figure it out, but it was a real pain.

[c150L]

Finally got this whole fuel delivery system figured out. I jumped power to the tank selector switch connector and found neither low pressue fuel pump in either tank would run. Today, the rear pump would run, put in more gas but still no sustained runs. Removed the selector valve/canister. This is mounted in the frame rail between front tank and the high pressure fuel pump. This canister switches itself depending on which tanks low pressure pump is supplying it. Tore it apart and found it still in the front tank position and the canister type filter on the bottom is plugged. Have to get a new filter. I also set the top section to supply/return from/to the rear tank. This is the one that the pump is working in. Think that perhaps the rear pump is weak and/or there was a big bubble in the system and with the plugged filter, the valve would not switch switch itself. In my cheap, one step at a time deal, I'm putting it back together as soon as I get a new filter to see if it will sustain running on the rear tank. Then I'll work the problems out of the front tank. If the canister valve won't switch from front to back,then I'm going on the belief that the rear pump is weak and needs a change out.

[nighthog]

I've got an old 85 factory service manual a friend of mine gave me this last week. The diagram I have shows a fuel reservoir canister followed by a 6 port tank selector valve. The tank selector valve is energized by the tank selector switch which then supplies power to the low pressure pump in the fuel tank being selected. If the selector valve is bad you won’t get any power to the low pressure pump. If you have a clogged filter crud could have shorted out the circuit or caused the motor to burn out if the valve jammed. If this isn’t the way your fuel system is set up all I can say is GOOD LUCK. Otherwise, I do have a diagnostics chart.

[badboyhighlander]

I have an 86 F150 and have dual tanks and my front tank fuel gauge is not working but I can run forever without having to fill up. Is the front tank pulling from the rear tank or are they seperate. Also when I try and fill up the front tank it always appears to be full when adding from the pump. The rear gauge does work.....and works normally.

Thomas

[chubbygoatboy]

I just did a master cylinder replacement, finished it, then the truck would not start. What the heck? If I turn on the key, I get no fuel pumps running. If I remove the relay, and jumper the two big wires together, the fuel pump runs and so does the truck! So I was sure it was the relay. I put in a new relay, and I still have no pumps running. If I remove the relay and jumper those two again, It will run with 42 psi on the fuel rail. The previous owner had both fuel tanks replaced, but only the front tank pump replaced a few years ago. He told me the rear tank pump has not worked in years, only run the front tank. Why would I suddenly loose all fuel pumps? ANY IDEAS are greatly appreciated!!

[Donut Dunker]

some of those have a filter in the bottom of the switch valve ( located on the frame) and it will get plugged i change my filter once a year

[fordracer1988]

could some of these also be the reason my fuel guage is not working also cause i just bought this truck not to long ago and the rear tank is shot i have no idea were to get a new one but i need it bad