Ok heres the story. Went out hunting last weekend and parked my 94 f-150 5.0 engine with dual tanks (figured you might want to know that to help me). Anyways when i got back to my pickup it wouldn't start, just cranked and cranked and cranked. For some reason i decided to switch from the front tank to the rear tank and it started right up. I thought that my fuel pump had went out on my front tank and was going to order one for it this coming week as when i turn the key on when switched to the back tank it would pump but when i turn the key on when switched to the front tank it would click at first and then did nothing. So last night i drained the front tank and filled the rear tank up completely, actually almost overflowing. Well i went for a drive after that last night and for some reason switched back to the front tank and it started working so i thought maybe it was ok and decided to fill it. This morning i was out hunting running on my front tank when i parked to look over some crp and when i went to start my pickup there was once again nothing, switched tanks and it started right up. Well i decided to just run off my rear tank for the rest of the day and deal with the front one later. I drove about ten miles on my rear tank and looked down at the gauge and it was already down a quarter tank!!!!! Drove to my brothers to see if i could figure out what was wrong. When i got out all i could smell was gas so i climbed under the pickup when i noticed the gas was somehow going from my back tank into my front tank and the front tank was now overflowing with gas!!! I tried taking the cover off the front tank and all it did was spray everywhere so i quickly replaced it. HELP!!!!!! I want to go out hunting again. lol

 

That sounds like the switching valve is leaking back. When it is leaking you probably don't have enough fuel pressure to get running and that could be why the motor will not start.

 

I fixed the problem by replacing the pump. I guess there is a check ball in the pump that closes the return line when the pump is off and it was stuck open when the pump quit and all the fuel from the rear tank that was supposed to recirculate back into the rear tank was dumping into the front one. Seems the switch in the cab just switches what pump gets power and the check ***** take care of the rest.