On My 1986 f350 front tank doesnt work. every time i switch over to it, the gauge stay on the rear tank and pulls fuel out of the rear tank. It also sends the fuel return back to the front tank causing it to overfill and spill out. i have replaced the selector valve by the fuel tanks, the connector going into the selector, and the switch in the dash. (i have a parts truck that i pulled these parts from and i know they where working just fine before i pulled it off) anything else can it be? im kinda at a loss sense i have replaced they whole system pretty much. could it be the in tank unit? my understanding is that thats just a pickup tube and fuel gauge.
Lemme see now.
You changed the FSV, Dash switch, FSV wire harness but still have the same problem.
There is no way the Dash tank selector switch could have caused this problem.
The wire harness to the FSV could not have caused this problem, providing it was wired correctly.
The front tank fuel level sensor cannot cause this problem.
The rear tank fuel level sensor cannot cause this problem.
So, the original problem must have been that the FSV didn't complete it's cycle and is stuck in mid position. This is usually caused by debris being picked up, more than likely the pickup screen has disintegrated.
Changing the FSV and all the other components, you now have the same problem.
The "NEW" FSV must have picked up some debris and got stuck, or the fuel return line for the rear must be connected to the front return port or both situations, since you say the front gauge don't work and still shows the rear tank.
When switched to the front tank, did the fuel gauge ever show the front tank fuel level?
Your going to have to drop the rear tank and inspect it for debris.
