I went through some serious******with my 1990 E350 Winnebago Warrior with the same problem. Not sure you will have it with 1991 Ford but if so I can spare you some trouble.
My fuel pump burned up in my RV about a year and a half ago. The trip before that the solenoids and fuel safety cut-off switch burned up. This must have been a precursor to the pump itself burning up.
When I say "burned up", I mean "burned up". The entire top of the electric motor was burned/charred away, and the spade connector that went to it was a molten metal blob. It is amazing that this can happen in a tank full of gas with no fire. But that's lack of oxygen for you, I guess.
I had it towed to a shop, and they replaced the pump via my VIN, and my gas gage no longer worked. We assumed that something electrical had happened to it and burned it up. My home mechanic found the real problem:
They installed the wrong fuel pump.
Ford changed fuel pumps mid model year in 1990. Check the VIN number on your CHASSIS, not the RV.
The problem is that the shop went to Auto Zone, and Auto Zone's computer did not know about the change and got the wrong pump. The pump looks fine and will install, but the problem is that the sending unit is reversed. So the gage reads backwards.
I was very close to suing the shop that installed the wrong fuel pump as they insisted it was not their fault. I said for $800 that I paid to have the vehicle fixed correctly it certainly was not *my* fault. However as I got to digging to get my paperwork together to sue I found that there really was no way this guy could have known. Unless he had bothered to go directly to Ford and do some serious investigative work. But who would not trust what the AutoZone computer said? And even if you went to the Airtex web site, their own product configurator spit out the wrong part number for my VIN!
Eventually the Airtex people sent me a replacement pump for free. But I had already spent $800 having the pump replaced ,and then another $300 when my home mechanic debugged the fuel gauge discovering the real problem. It will cost another $300 to have the pump replaced even when I provide the replacement pump. So I have not done it yet. Right now we gauge gas by the mileage rather than the gauge.
So. Moral of the story: Be careful and be certain when you are looking for the right fuel pump for that 1991 Ford! And if, after replacing, your gas gauge seems broken, they put in the wrong pump! Make sure they make it right.!
Steve
