Free Auto Repair Advice by Certified ASE Technicians
How Does it Work /
Engine Thermostat
How an Engine Thermostat Works
Inside your car's engine, thousands of controlled explosions called
combustion events caused by igniting fuel/air mixture inside the engine
generate heat. If this heat is not controlled the engine will overheat and
internal damage can occur. These high temperatures are controlled with the help
of the cooling system. A cooling system consists of a water pump, thermostat,
radiator hose, hose clamps, radiator, radiator cap and coolant. The thermostat
is designed to control the flow of coolant through the cooling system while the
engine is warming up to operating temperature. An engine needs to operate at a
particular heat range to be efficient. Once the engine is warm the thermostat
will open to allow coolant flow to maintain a particular temperature. Most
thermostats are designed to open at about 195° F but other
temperatures are available for a variety of applications. A thermostat consists
of a main housing, a plunger style of valve with return spring and a temperature
sensitive wax filled plunger that acts as the sensing and activating device. A
thermostat maintains engine temperature as is opens and closes throughout the
engine operation.
Engine Thermostat
When a thermostat malfunctions it can stick in the closed
position not allowing the coolant to circulate, causing the engine to overheat.
This will cause the vehicle to overheat in a very short amount of time (about 5
minutes). Or the thermostat could stick open causing the engine to run too
cold. In this case the service or check engine soon
light could illuminate, followed by a trouble code.
To test a thermostat remove unit. Prepare a pan of water deep enough to cover
the thermostat completely. Next install a temperature gauge
into the water along with the thermostat. A cooking thermometer works well for
this. Next, start heating the water while watching gauge, the thermostat should
remain closed until the water reaches 190° at this point the thermostat should
start opening and be completely open at about 195°. If the thermostat stays
closed through the boiling point the thermostat has failed and needs replacing.
If the thermostat is stuck open or broken it has failed and needs replacing.
Never run an engine without a thermostat because the thermostat works as a
system flow regulator as well. What this means is the thermostat has a specific
opening that regulates the flow through the cooling system. If the coolant is
allowed to flow too quickly through the radiator the coolant will not have time
to transfer the heat it has absorbed. This will cause the engine to overheat.
Typical Engine Cooling System
Engine coolant is used to transfer heat from the
engine to the
radiator by the
cooling system. The radiator removes heat from the coolant by forcing air
through the radiator fins. Without coolant your engine will overheat and
if left unattended severe engine damage will occur. Coolant colors can vary
from green, orange, blue and yellow each having their own protective properties.
(Note: coolant and antifreeze refer to the same product, in
below freezing, coolant lowers the freeze point hence the name anti-freeze and
in warm weather coolant helps raise the boiling point, "coolant")
Red and Green Engine Coolant
When a thermostat fails it will
either stop the coolant flow at operating temperature "stick closed" or fail to
stop the coolant flow causing the engine to run cold longer than necessary. If
the thermostat fails "open" it will cause the coolant to continuously flow
through the engine causing a
diagnostic trouble code (check
engine light) to be set by the computer. When a thermostat sticks closed it
will cause the engine to overheat quickly, usually within 5 to 15 minutes of
operation. To check for either of these conditions, drain coolant and remove the
thermostat (under the thermostat housing) if you are unsure of the location of
the thermostat on your engine consult a
car repair manual. Once you have removed the thermostat inspect the
condition of the main body check for any cracks or broken pieces and check the
valve to make sure it is closed. If the valve is open the thermostat has failed
and needs to be replaced. To check the operation of the thermostat prepare a pot
of water on the stove deep enough to cover the thermostat completely. Place the
thermostat in the pot of water, turn the stove on a medium/high flame, the
thermostat should open right before the water comes to a boil. If the water has
boiled and the thermostat valve is still closed the thermostat has failed and
needs to be placed.
Basic Checks
Low levels of coolant can lead to problems related to engine overheating. Always
maintain proper levels of coolant in the overflow coolant reservoir tank. ONLY!
Check the radiator when the engine is cold. Only add antifreeze and reinstall the
cap properly. Always drain and refill your
cooling system regularly.
How Does it Work
/
Engine Thermostat
Check Manufacturer Specific
Thermostat
Questions and Answers
Related Subjects
Got Car Repair
Questions
we will answer it for FREE!
GET
CAR REPAIR
MANUALS
- Get instant access to your vehicles drive belt routing, wiring diagrams,
trouble code information and other available updates and factory bulletins.
| 
