1991 ford f150 problem

I have an 89 F150 with dual tanks. You might search the group about your problem because I recall tossing in some ideas about ten years ago (which the recipient acknowledged helped).

With dual tanks you have no less than three fuel pumps - one in each tank and one delivering high pressure to the injector rail. When you select a tank you activate its fuel pump. A purely mechanical gadget called the 'dual function resevoir' feeds the high pressure pump.

All this is explained in the workshop manual.

An understanding of the system is necessary to have any hope of fixing the problem. And it is possible they changed the system between my model and yours. But probably not because it was simple and reliable. They may well still use it - they could do worse.