Fuel gauge stuck on full

Yellow with White marking on the wire from the guage to the selector switch. http://ww2.justanswer.com/uploads/sdcrowder/2011-06-10_002304_86_f150_fuel_tank_selsecter_switch_wiring_diagram.gif

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check wire from sending unit on tank. If it has any bare wire that is touching metal and grounding out, this will cause the gauge to peg on full. If the wire on the tank sending unit looks good then more than likely, the gauge in the dash is bad.

Fuel Sensor on one of the tanks

Do you have duel tanks or singel tank? it is possible the fuel filter is plugged. Thats the first thing I would check You could also remove the air cleaner and have someone crank it over and being careful not getting into the belts and fan open the plate in the throttle body and see if fuel is comming out of the injectors. If the fuel does not come out the injectors try replaceing the filter first.

It is most likely the sending unit in the fuel tank,but,check it first,if you unplug the connector to the fuel tank,the gauge should go past full.If you use a test light,and ground the clip on the end of the test light to a good,known,ground,and touch the right wire,it will light up dim(test light)and the gauge will pull down slow,to ,past the empty mark on the gauge,if it does this,the sending unit is bad,if it does not respond to any of this testing,then the gauge,or a connection to the gauge is bad.

in driver side of frame before the high pressure pump or lift pump.

you have a tank switching valve under the truck that may be bad,also check the fuel lines coming out of the tank they could be cracked and sucking air instead of fuel

yea fords with two tanks are known for getting stuck if you dont switch the tank constantly. you have to get to the valve that switches the two tanks, and free it up. it definitley isnt anything but the valve.

The switch that you use to change tanks is probably faulty - the switch supplies power for the sending unit in the tank, and to the transfer switch for the gas lines - use a meter to check it

i have the same problem and i replaced the shuttle valve and i fixed the problem it is located on the drivers side frame rail just behind the the main high pressure fuel pump it should have 6 lines going into it and out it is a mechanical part so its not repairiable hope this helps