Large variations in temp gauge are often due to pockets of air trapped in the cooling system. Lower coolant level so radiator is 1/2 full and let car warm up with heater running full blast. Watch water flow in radiator to determine if water pump and thermostat operate properly as engine fully warms up. As air is purged start refilling radiator with motor running at idle until 3" below filler neck then install radiator cap before shutting off engine or coolant can burp out of radiator upon shutdown. Let cool down, then check level in tank, top off to 3" below filler neck then check to see if temp gauge operates differently.
Last edited by Safe-T-Track; 06-16-2019 at 09:17 PM.
Reason: added info