Why does thermostat look like car will overheat

Hi @dd21 ,

You don’t state the model or year of the vehicle but it sounds like you have a sticking thermostat in the engine’s coolant system. (I presume that you mean the dashboard temperature gauge when you say thermostat).

The dashboard temperature gauge will rise as the engine warms but there is no coolant flowing through the engine (or the heater) until the thermostat opens at a pre defined set temp. As your thermostat may be sticking closed, your temperature gauge continues to rise.

When the thermostat finally opens the coolant will flow through the engine, radiator etc cooling the engine so that the temperature gauge will now fall. Warmer (hot) coolant will now also flow through the heater radiator in the cabin of the vehicle, (if it is switched to heat) that is why there is now hot air.

With a good thermostat usually you will not see the gauge fall back as the thermostat will open correctly at the normal operating temperature (for the vehicle) and the gauge will have reached the normal operating level and stayed there when it opened (with normal conditions etc)

Check online (YouTube is a good place to look), for how to replace the coolant thermostat in your year and model Oldsmobile.