I know this topic has been talked to death, but I couldn't find my exact issue.

I have a 1991 F-150 4x4, 302, 5 spd with dual tanks. When I bought the truck a few weeks ago the previous owner told me that the previous owner to him (he only had the truck 6 months, lost his job and had to sell it to me) told him the rear tank didn't work. He said that he hadn't really messed with it, but he did have to replace the fuel pump in the front tank.

The front tank only shows 3/4 full on the guage when the tank is completely full. Once the tank is down to 3/4 the guage works correctly. Not a big deal, just odd.

The rear tank is the odd one. I filled the rear tank and it seemed to work fine until I got to 1/2 a tank. The truck started chugging and I flipped to the front tank and the truck runs fine. If I flip back to rear the truck is clearly not getting the right amount of fuel.

Anyone have any ideas of what could happening?

Thanks!
Rich

 

Rich,

I'd bet the front tank sending unit is not working right. When I replaced my front tank sending unit a few weeks ago I dusassembled the old one. With a volt/ohm meter I tested the entire metal strip that the float contact touches. The results were crazy. Then I reconnected the sending unit to the harness and worked the float while watching the fuel gauge on the dash. The fuel gauge would show full when the float was at half, three quarters when the float was at full and empty when the float was at one quarter. Just wierd readings. So I replaced the sending unit and pump. Now the gauge is reading correctly at all fuel levels and I don't have to worry about the pump going bad(hopefully) for a while.

Sounds like you rear pump is going out. If when you switch to the front tank you have no problem then the high pressure pump on the rail is doing its thing correctly and maybe the pump in the tank is starting to go. However, it could just be the filter sock at the pickup inlet of the tank pump that has crap in it. You got to drop the tank and remove the sending unit/pump assembly to to find out and fix.

Sometimes it's nice to have two tanks for just such a situation. You can still use your truck while you do the fix.

 

He does not have a "high pressure pump on the rail", just the two high pressure pumps in the tanks with their output going to a "T" and then to the fuel manifold.

 

Whoops, well scratch the high pressure pump part of my theory.

On my '88 f250, my logic is probably more sound.