Engine Overheating Repair

Inside your car's engine, thousands of controlled explosions called combustion events occur. These explosions are created by igniting a fuel / air mixture inside the engine. Spark plugs are used to ignite the fuel / air mixture contained in the cylinders. These explosions are converted into power through the engine while producing a large amount of heat. These high temperatures are controlled with the help of the cooling system. A cooling system consists of a water pump, cooling fan, thermostat, radiator hose, hose clamps, radiator, radiator cap and coolant. 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 cooling fins.

Without coolant your engine will overheat and if left unattended severe engine damage will occur.  Engine coolant colors can vary from green, orange, blue, clear and yellow each having their own unique protective and environmental properties. Coolant leaks are a common car problem that can lead to overheating; we have listed some of the most common causes below. (Always inspect the engine cold, coolant is under pressure and can burn you). (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).

Troubleshooting Procedure

Check Engine Coolant Level, Test For Leaks and Test Radiator Cap - Engine coolant is used to transfer heat from the engine to the radiator; if a coolant leak is present the engine will eventually overheat. Inspect the engine coolant level in the coolant reservoir tank; coolant level should be between the hot and cold marks. Always check the coolant level when the engine is cold, preferably over night. If the coolant level is not between the reservoir marks the cooling system may have a leak.

Radiator Cap and Coolant Reservoir
Radiator Cap and Coolant Reservoir

If engine overheating has occurred the coolant level will naturally be low due to expansion of the coolant from the extreme heat of the engine. This heat expansion forces coolant out of the radiator and coolant reservoir. To test for an engine coolant leak move the car to a dry smooth surface and allow the engine to cool. Remove the radiator cap and carefully (do not spill) add water until full, then re-install cap. Start engine and allow to run for about three to five minutes (do not allow to overheat) while the engine is running inspect the ground below the engine, if an engine coolant leak is present observe the location of the coolant drops, this will help determine where to start looking for the coolant leak (shut the engine off before inspecting).

Leaking Radiator and Radiator Hose
Leaking Radiator and Radiator Hose

If no coolant is observed two additional checks are needed for a complete test. With the engine off remove the engine oil fill cap and turn it over, if a milky oil condensation is present the engine may have a failed cylinder head or intake manifold gasket allowing coolant to leak internally. To inspect engine gaskets disassembly is required. Next, the car heater core must be inspected; the quickest way to check the heater core condition without removal the heater core is to inspect the passenger's side foot well compartment carpet for the presences of coolant. If coolant is present the heater core has failed and must be replaced or repaired. After necessary repairs have been made refill the cooling system with manufacturers recommended engine coolant and recheck operation.

Check Engine Thermostat - An engine thermostat is designed to regulate the flow of coolant from the engine to the radiator. This temperature sensitive valve is designed to open when the engine has reached operating temperature (190°-198° F). The operating temperature of 190°-210° F is used to help facilitate fuel combustion. When a thermostat fails it will either stop the coolant flow "stick closed" and overheat or fail to stop the coolant flow causing the engine to run colder longer than necessary. If the thermostat fails to "close" it will cause the coolant to continuously flow through the engine creating a diagnostic trouble code (check engine light) to be set. 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 thermostat, (thermostat is located in 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 cranks or broken pieces, also 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 top deep enough to cover the thermostat completely. Install 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.

Engine Thermostat
Engine Thermostat

Inspect Engine Cooling Fan Clutch or Electric Fan Operation - The engine cooling fan system in your car is designed to move air through the radiator when the vehicle is at slower speeds or stopped. This air flow removes heat from the coolant created by the engine using the radiator as a conductor. Most radiator cooling fans are powered by the engine or by electricity. When a cooling fan fails it causes the coolant to retain heat, forcing the engine to run hot and eventually overheat. A cooling fan that is powered by the motor is engaged and disengaged from operation by a temperature controlled fan clutch. This fan clutch is constructed using a silicon grease and temperature sensitive coil spring that expands and contracts with heat. As the spring absorbs heat it expands engaging the clutch fan utilizing engine power to drive the fan.

A clutch fan can fail one of two ways, it can either lock the fan to the clutch causing poor mileage while producing a whirring sound, like an airplane is taking off next to you. Or the silicon grease can start to leak causing the fan clutch not to lock up allowing the fan to "freewheel", failing to pull air through the radiator when needed. To check for this condition the engine must be off, inspect the fan clutch for leakage at the front or rear of the unit (input shaft and temperature controlled expansion spring). If leakage is observed the fan clutch has failed and needs replacing. Next, take the fan blade and turn it, the fan blade should free wheel, if you cannot turn the fan blade the clutch has locked up and needs to be replaced.

Engine Fan Clutch
Engine Fan Clutch (fan blade not attached)

To inspect an electric motor cooling fan start with ignition key switch off, next spin the fan blade by hand, it should "freewheel" if the fan motor does not spin freely it has failed and needs to be replaced. If the fan motor "freewheels" the electrical system that operates the fan needs to be tested. Start the engine and turn the air conditioner to the coolest setting, if your car does not have an air conditioner skip this step. Within five minutes of the car air conditioner operating the cooling fan should activate, if not the fan motor fuse must be checked first; if the fuse condition is ok the remainder of the electrical system must be checked.

To further troubleshoot this problem a wiring schematic is needed which is located in a repair service manual. Use the wiring schematic to trace the circuit through the fuse and relay components, if everything tests ok the fan motor has failed and needs to be replaced. The cooling fan motor failure is the most common repair.

Radiator Cooling Fan
Radiator Cooling Fan



Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-10-15)