I have a 1991 f150 4.9l 300 inch 2x4. The seller told me that the rear tank does not work and they do not know why. I start it on the front take and hit the switch and it dies within 2-3 seconds. Is this the fuel pump or some other problem?

 

More than likely it will be the pump but you need to do a voltage test at the pump to make sure it is getting power.