Got a car engine that is overheating? When you see your engine temperature
gauge on the hot side or see a warning light on or smell something hot stop the engine immediately. We are a team of ASE certified
mechanics that have created this guide for you to repair the overheating problem yourself
or see what you paying for when taking your car into a garage to be fixed.
What Goes Wrong?
The engine cooling system is designed to help the engine maintain a
particular operating temperature of between 205° F
and 220° F (96°C - 104°C). When the system leaks or has some kind of mechanical
failure it will allow the engine to run over the recommended temperature which
can cause major engine damage such as:
- Blown head gasket
- Cracked cylinder head
- Cracked engine block
- Seized pistons
- Collapsed piston rings
How Does it Work?
The cooling system is a pressurized, sealed circulation system that consists
of eight major components:
- Radiator - Reduces temperature of the coolant for the engine
- Water or coolant pump - Circulates coolant throughout the engine and radiator
- Thermostat - Stops the coolant from flowing until the engine is warm
- Coolant or antifreeze - Acts as a cooling agent and provides freeze and rust protection
- Coolant Hoses - Transfers the coolant from the engine to the radiator
- Radiator cooling fan - Draws air thought the radiator for cooling
- Pressure cap - Holds pressure on the cooling system
- Reservoir - Holds expansion coolant when the engine gets hot and then allows to coolant to be drawn back into the system when cold.
The engine operating temperature is maintained using a thermostat which is designed to
stop the coolant flow when the engine is cold. Once the
desired temperature is obtained the thermostat opens automatically allowing the
coolant to flow into the radiator and cool the engine. Coolant is used to
transfer heat from the engine to the radiator to maintain the engine
temperature. Coolant is also used to
prevent corrosion which can damage internal engine parts such as gaskets and
block and cylinder head castings.
A cooling system is held under pressure at about 16 to 18 psi while the engine is at operation
temperature. System pressure helps the coolant gain a higher boiling temperature one degree for every two pounds of pressure. For example; when a cooling
system is held at 18 pounds of pressure it raises the boiling point by 9 degrees.
Let's Get Started!
Begin with the car parked on level ground with the
parking brake set. The following steps are presented in order of popularity.
Step 1: Check the Cooling System Pressure and Level
The cooling system must maintain pressure or the engine will overheat. The easiest way to
check the operation of the cap is when the engine is warm grasp the upper
radiator hose and squeeze it. You should be able to feel pressure in the system.
When the engine is cold no pressure will be present in this case you can remove
the cap for testing. If there is no pressure present the system has a leak or
the cap has failed and needs replacement.
To check the coolant level start by observing the level that is in the coolant
reservoir. If the coolant level is a little low it will need to be added to
which is normal over time. Slowly remove the reservoir or radiator cap,
a small amount of pressure maybe released which is normal. If the reservoir is
empty the system could have a coolant leak
or the system is just extremely low. Some cars use the coolant reservoir cap to
hold pressure on the system while others depend on the radiator cap to maintain
Step 2: Check the Cooling Fan Operation
A radiator cooling fan is used to move air through the radiator cooling fins which helps cool the engine. If this fan stops
operating the engine will overheat or run hot. The cooling fan is located directly behind the radiator and is used to pull air
through the radiator at lower vehicle speeds to keep the engine from overheating.
Cooling fans can either be mechanical which are belt driven by the engine or electric
motors mounted on the radiator both are controlled by temperature.
There are two ways to check an electric motor cooling fan. Start with the
engine and ignition key switched off and spin the fan blade by hand, it should
"freewheel" and spin easily. Next, start the engine and turn the air conditioner
to the coldest setting. Within one minute of the car air conditioner operating
the cooling fans should activate. If not the fan motor fuse must be checked
first. If the fuse condition is okay check the cooling fan relay. If these items
check out okay the
fan motor has gone bad and needs replacement
. Cooling fan motor failures
If your car is not designed with an electric fan it will have a thermo fan
clutch which is attached to the water pump and driven by the engine. This device
engages and disengages the fan blade via a temperature sensor at the center of
the clutch. When this component goes bad it fails to "lock up" which diminishes
the fan blade performance causing the engine to overheat by allowing the blade to freewheel.
When the engine is hot this clutch should engage being followed by a roaring
sound as the fan starts to work. If the engine is hot and you can see the blade freewheeling
the clutch fan has gone bad and needs replacement.
Step 3: Replace the Thermostat
The thermostat will not allow the coolant to flow into the radiator when the engine is cold.
When the thermostat goes bad it will do so by sticking closed. This means the
coolant never gets a chance to be circulated through the radiator. When this
failure occurs the engine will overheat rapidly usually within the first fifteen
minutes of driving and can be accompanied by a thumping sound as the hot coolant
is trying to mix with the cold coolant in the radiator. Do not open the radiator
to check the coolant level allow the
engine to cool
then replace the thermostat
Step 4: Checking the Radiator
The radiator is responsible for dropping the temperature of the engine coolant
and can fail in a few different ways. First is external blockage, because your
car goes down the road which is constantly bombarded with debris such as; dirt,
hair, leaves and bugs to name a few, the cooling fins become plugged stopping
air travel through the radiator core causing the engine to overheat. This
condition can be hard to detect because the air conditioner condenser sits in
front of the radiator. Use a flashlight to help you see in between them from the side or bottom bottom. You can use a garden hose
to pressure wash the radiator from the backside facing forward (back flush) to
help alleviate this problem.
With the engine cold remove the radiator cap and inspect inside. You may need to
drain some of the coolant out or use a flashlight to see the ends of the radiator core tubes.
In the example below the tube is clear with no corrosion around the end of the
tube. When a radiator plugs internally these tubes become clogged with calcium
stopping the coolant flow which inhibits the radiator's cooling ability. When
the cooling system has been neglected this condition will occur and cause the
engine to overheat in which case
radiator must be replaced
. This condition generally occurs
gradually over time and will be more noticeable when climbing a grade or in warmer
Step 5: Check the Head Gasket
A head gasket is used to seal the combustion process within the engine block
and cylinder head. This gasket is prone to failure due to natural engine block and cylinder head
expansion. Heat and corrosion can cause failure which is produced by a process called
electrolysis. This can cause coolant to enter the engine and exhaust gasses to
be forced into the cooling system which displaces the coolant from the system.
This problem is becoming more popular as engine components such as engine blocks
and cylinders heads become more lightweight. Fortunately to
test for a blown head gasket
is a simple process. When this gasket fails
it can also cause a rough running engine at start up.
Step 5: Checking the Coolant Pump
The water or coolant pump is a mechanical pump which is responsible for circulating
coolant throughout the radiator and into the engine which is driven by the
engine serpentine or timing belt or by the timing chain. On most hybrid gasoline
engine's this pump is powered by an electric motor. When a water pump fails it can
do so in one of two ways. First it can cause a leak which is what happens when
it's shaft seal fails or when the shaft bearings fail destroying the seal. This
can sometimes be accompanied by squeaking, squealing or a rattling sound.
A water pump will typically last between 50,000 and 80,000 miles.
In some instances the impeller of the pump will become dislodged stopping the
pumping action. There are only two way to check for this problem, the first is
open the radiator and watch for the coolant to flow inside the radiator while
the engine is at operating temperature. The second and more difficult way is to
drain the system and remove the pump for a physical inspection.
Step 5: Transmission Cooler
Automatic transmission vehicles utilize a fluid cooler inside the radiator to
help remove heat from the transmission fluid. When this cooler fails it will
allow transmission to contaminate the coolant inside of the cooling system which
inhibits the heat transfer of the coolant. This problem is easy to check for, simply open
the radiator or reservoir cap when the engine is cold and look for a pink milky
substance. If found the system will need to be drained and flushed along with a
new radiator installed. As a side note if this condition exists the transmission
fluid may need to be flushed and changed as well. This is because the radiator
maintains its pressure when the engine is shut off. In turn the transmission fluid
pressure drops to zero which allows coolant to be pushed into the transmission
fluid lines and into the transmission.
Step 6: Engine Oil Cooler
Some engines are designed with an engine
oil cooler to reduce heat by way of the engine oil. This is achieved by running
engine coolant through the cooling fins of the cooler to reduce heat. When
this cooler fails it can cause engine oil to be released into the cooling system
in much the same way as when a transmission cooler fails and can be checked in
the same fashion but instead of pink milky goo it will be tan or brown in the
reservoir or radiator in which case the engine oil and radiator coolant should be changed along with
the engine oil cooler.
Step 7: Catalytic Converter
When a catalytic converter starts to fail it can partially plug up which causes the engine to
have less power. To compensate for this low power condition the driver will
increase the throttle which creates excessive heat overloading the cooling system.
This condition is always be accompanied by low engine power. To
test the cat converter use a infrared temperature
tester to monitor the
ingoing and out going temperatures.
Car manufacturers have found that a 195° thermostat is optimum for efficiency.
Below this threshold the catalytic converter will not work properly which will produce
increased emissions. In the early years of automobiles water was used for the cooling system.
Water is the most efficient fluid to absorb and dissipate heat. The disadvantage
of using water is that it freezes at lower temperatures and causes rust inside the engine.
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Article first published 2017-01-27