Easy guide on how an automotive cooling system thermostat works, this information
pertains to most vehicles.
An engine cooling system thermostat is designed to stop the flow of coolant in
the cooling system when the engine is cold, this is done to help the engine warm
up to optimum operating temperature which improves efficiency and emissions output.
Once the engine heats to 195° F or 96° C it will open allowing the radiator to remove
heat from the coolant so it can be circulated back into the engine. These units
are located either at the upper or lower radiator hose connections.
A housing is used to hold the thermostat in place while allowing the radiator
hose to be connected and keeping the system circulating.
Engine Thermostat w/ Housing Removed
A thermostat is a normal service item that when fails will either stick closed
making the engine overheat quickly, or stick open causing the engine to warm up
to slowly which will trigger a check engine light.
Thermostat Housing - Hose Disconnected
An air bleed is located at the top of the thermostat to allow air to escape from
the system while being filled, a plunger is the actual valve that opens and allows
A gasket is used to seal the thermostat housing to the engine block or intake
Air Bleed - Plunger
Prepare a pan of boiling water deep enough to cover the thermostat completely,
gently drop the thermostat in, observe the plunger, it should start to move and
open completely within 45 seconds, if the plunger doesn't move the thermostat has
failed, if the thermostat plunger is open whilst being removed it has also failed.
A thermostat maintains engine temperature as it opens and closes throughout the
engine operation. Once removed, inspect the condition of the main body of the thermostat,
look for cracks or broken pieces and the valve to be closed. Avoid running an engine
without a thermostat, this unit also acts as a system flow regulator, if coolant
is allowed to travel too fast in the system, the heat exchange rate is reduced in
the radiator and the engine will overheat. A thermostat consists of a main housing,
a plunger valve with return spring. Temperature sensitive wax fills a plunger that
acts as the sensing and activating device.
Article first published 2016-02-05