1992 F150 Fuel Tank Problems
I have a 1992 F150 4x4, dual tank, 5.0L, with fuel injection. There are two high pressure pumps, one in each tank. There is no third pump on the frame rail. There is no distribution valve on the frame rail.
Here is the problem. Recently replaced the fuel tanks due to rust/leaks. After the replacement the tanks were filled with fuel. I have been running the truck with the tank selector switch set for the front tank. The fuel gage needle does not come down off of full. However, when I switch to the rear tank, I notice that the fuel gage is down quite a bit. It seems the truck is drawing fuel from the rear tank, even though the selector switch is set for the front tank.
I have gotten under the truck while a helper turns the key to the on position, engine off. The front pump runs for a short burst when the key is turned on and the selector switch is set for the front tank. I then repeated the test with the selector switch set to the rear tank and the rear fuel pump runs for a short burst.
I then held my hand against each tank to feel the vibration of the fuel pump and after verifying the front pump is in fact running started the truck. I can feel the vibration of the front fuel pump running while the engine is running.
I repeated this same test with the rear tank. I also stuck a small tube into the tank and put the other end in my ear and can hear the fuel pumps running. First with the selector switch on the front tank the then with the selector switch on the rear tank. I have also verified that only one fuel pump runs at a time (i.e. both aren't running together).
I have removed the selector switch and checked it with a digital multimeter and the switch seems to function properly. I also jumpered the connector from the selector switch to select the front fuel pump and it does operate.
I am at my wits end with this problem. Does anyone have any insight as to what is going on? How do I further diagnose the problem? I don't feel like throwing a bunch of money buying fuel pump modules if that isn't the real problem. Please help.