fuel gauge

If the gauge needle moves from home position to full at switch on, and continues to operate in just that manner for any length of time,
the fault has to be the sender. If the sender wire was grounded to zero resistance the gauge would die in 5 minutes or less.
Once inside the sender I expect you'll find what was a round contact point simiilar to a BB shot is worn away to 2 hook like points and snagged on the resistor wire.
Prove me wrong, wont hurt my feelings :)
Good luck