As the title says, I have a 1990 Ford F-150 XLT Lariat, L6 4.9L 300. Keep in mind this truck has two fuel tanks, one pump, and one filter. It's sat for over 3 years before we bought it. We got it running and then drove it. It sounded great for how long it sat. But we were driving it up a hill and when we got to the top it died. We assumed since the gas must be super bad the fuel filter or pump was bad. Since then we've replaced the filter. But now when it sits it idles way higher than it should but then goes down. Yet, when it idles slower for a while it dies. However it gets weirder, if we switch the tank to rear it doesn't die and it runs fine. What are all y'all's thoughts?

 

I could be wrong , but I believe you are missing something.
If you listen at the back by each tank , you should hear the fuel pumps in the tanks run for approx 2-3 seconds when you first turn the key on.
There is a fuel pump assy (module) in each tank.

The better way to go about all this is to get hold of a fuel pressure gauge and connect it to the fuel rail on the engine.Watch the reading after you turn the key on.
It should go up to approx 35-45 lbs.
If not the cause should be determined. .We don't condemn the pump assy until all power and ground connections are checked and confirmed good.

 

Fuel pressure specs for 1987-1994- 4.9L sixes should be 45-60 psi off the rail's shrader valve.You can compare each tank's pressure to specs. Each tank has an internal low pressure pump and there is a high pressure pump common to both tanks mounted near the fuel filter on the left side frame rail.The erratic idle could be caused by a carboned up idle air control (AIC) pintle and its bore within the throttle body. It was a common problem with the 4.9.