Temperature Gauge Issue

Lift the wire off the sender and measure voltage from the wire to ground, it should be 5v. if the regulator is good. If OK use a resistor or variable resistor to simulate the sender resistance. Maybe google the span of resistance from cold to hot to find what
resistance is needed. Did the sender ever work? could be the previous owner installed a sender with the wrong range, or there is teflon on the sender threads preventing a good ground connection.