My 91 with dual tanks won't run on the front. It had before. When I switch from rear to front it studders a little and dies. Could the selector valve be the problem or am I looking at a fuel pump? It doesn't spit gas from anywhere or anything like that

 

You don't have a selector valve.

 

Really? So it's definitely the pump then? Is it only on the newer models then mine that had that valve?

 

'87-'89 has the valve. I would assume pump because you said it was working and both pumps share most of the electrical system to operate them.

 

Okay. I'll test the connection going to it then. What actually switches it over then on mine?

 

Inside each tank is what's known as an FDM (Fuel Delivery Module). This is made up of a high pressure electric fuel pump, a jet (venturi) pump, a shuttle valve, a check valve, and the fuel strainer and level transmitter on the outside of the FDM (which is of course submerged in fuel).

How the tanks are selected is completely electric. For example when you switch to the front tank, the electric fuel pump in the front FDM is energized and begins pressurizing the fuel line at the same time as the rear FDM pump is cut out (de-energized). Fuel is prevented from entering the rear tank by the rear FDM's check valve. As the fuel circulates thru the fuel rail on the engine and returns towards the tanks, it is directed into the currently selected tank by the shuttle valve on the active FDM (the shuttle valve on the FDM that is turned off will close by spring pressure and block fuel from entering).

That's why you see the posts from the 92-96/7 guys having one tank filling the other. The valves in the occasionally FDM go bad and allow fuel from one tank to leak into the other tank.

Pretty simple, and more reliable than the old setup.

 

Ok thank you guys for the info!

 

The switch.

 

JoeRome - there was a recall on the 90-93 F150's for your issue. For the recall, Ford would install a new exterior check valve on the outside of the tank, and install a fuel pressure regulator. The kits are hard to find, but I found one on e-bay. However, I decided that I'm just going to replace the tank and FDM anyway, so I don't need the kit. I paid $40 for it on ebay. If you want it for $40, I'll send it to you (I'll even pay shipping!!). Let me know. Here's a pic of the kit:

 

That fixes crossfilling not bad pumps.