My son just bought a 1990 F-150, 5 Speed, 4x4, 300 engine. We replaced the rear tank fuel pump, but the truck will still not run when the fuel selector is placed in the Rear Tank position.
Looking through the Haynes Maint. manual has not helped me much. I reaed thru the thread posted a couple years ago, but I'm afraid that wasn't much help either (or I'm too ignorant to understand it), so any help will be much appreciated!

 

Does the pump selected pump run for one second and stop when you turn on the key or does it run all the time with the engine not running?

 

Yes, it will run for about a second. We can start the engine, it will run for about two seconds, then die.

 

Sounds like the fuel system might be OK.
But first you can either put on a new fuel filter or get a fuel pressure gauge and hook it to the fuel rail and see if it dies from lack fuel pressure (Approximately 30 PSI).

Normal running fuel pressure is 32-34 PSI at idle and about 40 PSI at WOT.

 

Bill, thank you so much! We were thinking that might be part of the problem (fuel filter). The fuel line broke when putting the rear fuel tank back in. We rigged it back in with a splice, but wondered if that splice would restrict fuel flow.