Engine Thermostat

What it does

The engine thermostat is a spring-loaded valve about the size of a hockey puck that sits where the coolant exits the engine toward the radiator. When the engine is cold, the thermostat stays closed — this lets the engine warm up quickly. Once the engine hits operating temperature (usually around 195–200°F), the thermostat opens like a flower, allowing coolant to flow to the radiator and cool down. If it sticks closed, the coolant can't escape — the engine overheats. If it sticks open permanently, the engine never reaches full operating temperature — it runs too cold, gets bad fuel economy, and the heater blows lukewarm air.