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ASE PS2 Test Preparation Guide: Cooling System

By Larry Carley

The major components in the cooling system are the radiator, radiator cap, coolant reservoir, fan, fan relay, water pump, thermostat, hoses, belts, coolant sensor and antifreeze.

Coolant from the engine is circulated to the radiator by the water pump to get rid of the engine’s waste heat. The coolant usually exits the engine through the upper radiator hose, and returns through the lower radiator hose.

The engine’s operating temperature is controlled by a thermostat (usually located where the upper radiator hose connects to the engine). If the thermostat fails to open, it will cause the engine to overheat. The thermostat also can be damaged if the engine overheats due to other problems such as low coolant, coolant leaks, a bad water pump, clogged
radiator or defective cooling fan or fan clutch. If the thermostat fails to close, the engine may never reach normal operating temperature, which can prevent the engine management system from running in closed loop. This can cause increased emissions and fuel consumption as well as cylinder wear.

If a thermostat needs to be replaced, the replacement must have the same temperature rating as the original. Other items that may also need to be replaced when changing a thermostat include the thermostat housing and gasket, and radiator hoses.

Radiators in most late model vehicles are aluminum with plastic end tanks. Most older vehicles have copper/brass radiators. Both types are vulnerable to internal corrosion caused by coolant neglect. Leaks can sometimes be stopped by adding a sealer product to the coolant, but eventually a leaky radiator will have to be repaired or replaced. Replacement radiators are price competitive with repaired radiators, and can be upgraded for improved cooling capacity. The width, height and hose connections on a replacement radiator must match the original to fit without modifications. The replacement radiator should also have the same (or better) cooling capacity as the original. Cooling capacity depends on the thickness of the radiator, the number of fins and tubes and the efficiency of the fins and tubes. Increased cooling capacity is recommended for towing and performance applications.

Most late model vehicles have a plastic coolant reservoir, which is often pressurized. The reservoir typically holds up to a quart of coolant and prevents the loss of coolant if the engine overheats. The coolant level is maintained at the reservoir (never open a hot radiator cap!).

The radiator cap, which may be mounted on the radiator or the coolant reservoir, seals the cooling system. Pressure caps raise the temperature at which the coolant normally boils for added boilover protection during hot weather. System pressure ratings vary from 5 PSI to as much as 21 PSI, or even higher on some late model cars. A weak spring in the cap or a leaky seal can allow coolant to escape from the radiator, which may lead to overheating. The cap should be pressure-tested if the engine has been overheating, and replaced if it fails to hold its rated pressure. Replacement radiator caps must have the same pressure rating as the original.

The water pump is usually located on the front of the engine, though it may be on the back in the case of some front-wheel drive vehicles with transverse (sideways) mounted engines. The water pump is usually belt-driven. On some overhead cam (OHC) engines, the pump is mounted under the timing belt and requires considerable labor to replace. For this reason, you should recommend replacing the pump if the timing belt is being replaced for scheduled maintenance (recommended every 60,000 to 100,000 miles depending on the application). The service life of the water pump and timing belt are about the same so changing both at the same time can save the vehicle owner money on future repairs.
A slipping serpentine belt can reduce pumping efficiency, causing the engine to overheat. Pumps can also fail if the impeller blades inside the pump (some of which are plastic) become worn or damaged, the impeller comes loose on its shaft, or the shaft seal starts to leak. Bearing noise or play are also indications that a water pump is failing and needs to be replaced.

The bolts that mount the water pump to the engine sometimes thread all the way through into the water jacket. Sealer must be used on the bolt threads to prevent coolant leaks when the pump is replaced.

Serpentine belts also have a limited life, typically 50,000 to 60,000 miles for older neoprene belts, and up to 100,000 miles for newer belts made of EPDM. Older neoprene belts will often crack and fray as they age. Newer EPDM belts will usually not crack, but may fail from internal wear that weakens the cords inside the belt. Replacement belts must be the same length and width as the original.

The automatic tensioner that maintains tension on the belt may also need to be replaced if it is sticking, making noise or cannot maintain proper belt tension. Belt idler pulleys should also be replaced if noisy, worn or sticking.

The radiator and heater hoses also have a limited service life. Hoses that are hard, brittle, cracked or bulging need to be replaced. Replacement hoses must be the same length and diameter as the original, and some replacement hoses may have to be trimmed to length to fit a particular application. New hose clamps are recommended.

The electric cooling fan cycles on when the engine is hot, when the A/C is on, and/or when the vehicle is driving at low speed to improve airflow to the radiator for more cooling. Older rear-wheel drive vehicles may have a belt-driven mechanical fan with or without a clutch, and some import vehicles use a fan driven by hydraulic fluid from the power steering pump. Problems with the fan drive mechanism, drive belt, motor or motor relay, or the coolant sensor that operates the fan can all cause an engine to overheat.

The coolant that circulates inside the cooling system is usually an ethylene glycol (EG) based antifreeze/water mixture. Most late model vehicles are filled with some type of long-life coolant that uses organic acid technology (OAT) corrosion inhibitors. Silicates may also be added to some of these coolants for added protection of aluminum surfaces. When the coolant is changed or topped off, a coolant that meets the vehicle manufacturer’s requirements or is compatible with the original coolant should be used. All long life coolants should be changed after five years regardless, of mileage.


Steering & Suspension

Manual Transmission/Transaxle

Ignition System


Fuel System

Exhaust System

Engine Mechanical Parts

Emission Control Systems

Electrical System

Drivetrain Components

Cooling System


Automatic Transmission/Transaxle

HVAC Heating & A/C System

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