Where is the fuel relay swich on a duel tank f250

check for a resistance in the circuit to the pump
from your description when you have high rpms you will have max charge from the alternator which would increase the voltage available to that pump
bad connections , bad ground (earth)
tyr using a multimeter set on voltage to see the operating voltage available and if it increases with the rpms

There are several things that I would check, Richard. The most obvious is a fuel line leak from the rear tank. Another possibility is a kinked neoprene hose near the tank's fuel pump. It the rear tank has not been used for a while the tank may have some water in it. Try a bottle or two of water remover (isopropyl alcohol). If none of those things are found, then you should get a fuel pump pressure test.

You know, sometimes a pump goes bad and needs to be replaced. Have your mechanics test it

If you haven't already, it's best to get a schematic for this truck's fuel pump circuit.
Try your local library's website, you may be able to download it for free

You should have a cutoff swich under dash on pass side of truck.
The fuel pump a a 7.3 1997 f250 is behind the fuel/water separator.Between the cyl heads.Remove the turbocharger then fuel/water sep.remove banjo bolt and seal ringsat rear of pump.remove line asmby
one for each cyl head at rear.and remove fuel asmby.
Ensure camshaft lobe for pump is on base circle with lobe facing away
from pump to prevent pump damg.

The fuel pump relay and the oil pressure switch as well. The oil pressure switch runs power to the fuel pump when you first start it
When the oil pressure reaches operating range the relay takes over
They are both inexpensive so replace both of these parts to be safe and kill two birds with one stone. If one goes out the other will soon enough

first, is this a diesel? if it is there is a line or oring loose allowing it to **** air--diesels dont like leaks like that and there is a whole procedure for voiding the air from the lines on diesels. next could be the in tank pump and also does it have duel tanks? does the duel tank switch work? running out of fuel with in tank pumps can burn up the pump (the fuel itself provides lubrication and cooling for the pump, gas or diesel) and of course check the filter, running out and refilling could have disturbed trash in the tank

Hey lets try this ! Locate your starter solenoid & look for a yellow wire marked with a "W" . That may be the fusible link for fuel pump. Let me know what you find.

I would start by checking you switch for the tanks to make sure it is working right and if the front tanks fuel pump does not work after you check the switch I would get a 12 volt electrical test light and see if your getting power to the front fuel pump if your getting power to it I would say you have a bad pump . But some times the pump will run but the diaphragm gets weak in them.

Verify power coming OUT of the switch.
Verify power at the valve.

No power out of switch replace switch.
No power at valve replace wires.
Good power to valve replace valve.

Good luck!