my 91 5.0l has a low in the tank and a high on the frfame rail. and my 92 4.9l had the HP pump in the tank.

i'm sure that 92 was the year they switched.
that or it might be fords famous half year changes lol

 

Your right about the half year thing with Ford lol. Either way the OP should run a pressure check on the system after he pulls the codes. Just remember that the I6 runs a higher pressure than the V8's 50-60 psi vs. 35-45 psi

 

It doesn't have the 3 pump setup, it's only 2 since it's a 1991. I went to a mechanic and he gave me a new relay and thought that the relay that powered the fuel pump relay was bad, which is the only one we didn't check. He said he's seen that a lot and I know it's not a fuel pump because neither tank work and I haven't been running the back one for about a month because the gas gauge is messed up, so I think it's the relay. I will update when I check it out this afternoon.