[Davescort97]

I'd venture to say that the thermostat you put in is working correctly. Most likely the temperature sending unit for the gauge is faulty. Your car has 2 temperature sensors. One for the gauge and one for the computer. The one with 2 wires instead of 3 is the sensor for the gauge. I'd change it out and flush the cooling system and put in regular propylene glycol antifreeze (green stuff). GM just settled a class action lawsuit for Dexcool. Apparently it causes leaks in cooling systems. I wouldn't use it.