The thermostat is a valve in the coolant path. It stays closed on a cold engine so the engine warms quickly, then opens to let coolant flow to the radiator. A thermostat stuck closed makes the engine overheat quickly even though the radiator stays cool. One stuck open makes the engine slow to warm up and the gauge read low. The opening temperature is stamped on many thermostats and given in the manual. Draining coolant and opening the housing is usually a workshop job.
Safety
- Never open the cooling system on a hot engine.
- Hot water from the bench test scalds. Use tongs and keep children away.
- Coolant is poisonous; catch all of it.
What you can check safely
- Check the coolant level cold: Check and top up coolant.
- Start the cold engine and idle. Carefully touch the upper radiator hose every minute with the back of your hand. On a working system the hose stays cool for a while, then becomes hot quite suddenly when the thermostat opens.
- If the gauge climbs high but the upper hose and radiator stay cool, the thermostat may be stuck closed (or the water pump is not pumping, or there is an air lock). Stop the engine.
- If the engine takes a very long time to warm up and the gauge stays low on a cool day, the thermostat may be stuck open.
What the workshop does
- Drains enough coolant, opens the thermostat housing and removes the thermostat.
- Bench test: hangs the thermostat in a pan of water with a thermometer, heats it slowly and watches the temperature at which it starts to open and how far it opens. Compares with the manual. A thermostat that does not open or does not close fully when cooled is replaced.
- Checks the water pump at the same time: weep hole, bearing play and impeller. See Find a coolant leak.
- Refits with a new gasket or O-ring, refills and bleeds air.
What to ask for
- A thermostat of the correct opening temperature for your model.
- Do not accept removing the thermostat as a fix. Without it the engine runs too cool, wears faster and may still overheat because coolant flows past the radiator too fast in some designs.
Check your work
- Upper hose gets hot at the expected point, gauge settles in the normal zone, fan cycles in traffic.
If it did not fix the problem
- Still overheating: Liquid-cooled engine overheats.