I have a 1988 F-150 xlt lariat 4x4. it's pumping fuel to rear tank.rear rear tank is disconnected electrically.Is the front tank pumping into the rear tank?
I know on my old 95 F250 It did have a tube with a one way vaule into the back tank from the front tank.
There is a canaster with a filter inside just between the front and rear tanks. You can cut the lines and run just off the front tank. This solved the problem for me.
The canister is really exspensive and hard to find. It fails and prevents all fuel from going forward.
My 88 did the same thing. Its due to the diverter valve that controls which tank to pull fuel from. It isn't closing all the way so it pumps fuel to the rear tank while sending it to the engine. You should hook the rear tank back up and use your extra gas.mine would go into the front tank so I'd just run my rear tank for awhile then run my front tank when it was getting close to full.
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