When get into my truck before its started it reads 1/4 of a tank. It says 1/4 of a tank all the time. But when i start the truck, the gas gauge goes to empty. My rear tank doesn't work so i have to use the front one. What could be wrong with it?

 

More than likely the float has sunk in the front tank of your 1994 F150.
At any rate the wire leaving the fuel gauge has low resistance to ground.
If it is pegging passed empty then that wire has a short to ground.

 

Thanks! I'll check to see if that is the problem

 

Yes you can check by opening the wire to the tank, the gauge should go passed full and peg with an open wire.
The Fuel Gauge Sender's resistance controls the magnetic Fuel Gauge's pointer position. The Sender has a resistance of 145 ohms when the fuel tank is full and 22.5 ohms when the fuel tank is empty.

You can open the wire by unplugging C205 at the safety wall in the engine compartment.