93 F150 EFI/dual fuel tanks. I may have a front/rear selector switch problem or seemingly one pump is siphoning from the other tank. A few weeks ago I replace the rear fuel pump that was not pumping(truck would not get fuel to the fuel manifold). Just before it failed the "front" tank fuel gage stop registering.I filled the front tank up and the gas gage needle would not move. The rear would register although the pump went south. Today, while driving with in "front" tank switch selector mode, the truck began to stumble and I thought it was about empty and switched to the rear tank (which was about 3/4 full from a previous gas up, I had not used the rear pump/tank since then) and the fuel gage read empty and the truck stalled. I checked and found out the rear tank was empty!! Evidently the front tank/pump, while in the front switch mode must had siphoned the gas out of the rear tank? Am I looney or what. Is there a check valve in the line between the two tanks? I thought the fuel pumps contained the check valves. OR could somehow the electrical circuit within the front/rear selector switch become faulty and make the circuit to the rear tank pump ? I filled the rear tank up and the truck ran fine. I found the front tank was low by only about 2 gallons from the previous fill up! In the front tank/pump mode the truck would not start due to fuel, but once I filled the rear tank the truck would start when either the front or rear selector switch mode was pushed?Go figure..
I can't find a fuel or electrical schematic for the 93 F150 truck indicating dual pumps/tanks. Does anyone have info on a diagram? And most of all anyone can solve this phantom fuel problem. Thanks