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ASE P2 Test Prep: Cooling System

Information to help parts professonals pass the ASE P2 test.


The cooling system’s job is to manage heat produced by the engine, to help the engine maintain warm up quickly and maintain a consistent operating temperature, and to provide heat for the heater.

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The coolant is a mixture of water and antifreeze that circulates inside the engine and radiator. There are two basic types of antifreeze: ethylene glycol (EG), which is the most common, and propylene glycol (PG), which is less toxic to animals. Antifreeze is usually mixed in equal parts (50/50 mix) with water. With EG antifreeze, this provides freezing protection down to -34 degrees F and boilover protection up to 265 degrees F with a 15 psi radiator cap.

Coolant needs to be changed periodically to renew the chemical additives that protect the cooling system against corrosion. The recommended change interval for traditional antifreeze (green and yellow) is two years/30,000 miles, and for the long-life coolants it’s five years/150,000 miles.


Since 1996, General Motors has used a long-life coolant called Dex-Cool that is dyed orange. Long-life antifreeze and ordinary antifreeze should not be intermixed, as doing so will shorten the service life of the long-life antifreeze to ordinary antifreeze.

Rust, scale and sediment can be removed from a cooling system by using a chemical cleaner and reverse flushing the system when the coolant is changed. Leaks can be prevented and small leaks sealed by using a sealer additive.

The thermostat regulates engine operating temperature. It is usually located in a housing where the upper radiator hose attaches to the engine. The thermostat blocks the flow of coolant until the engine reaches a certain temperature (typically 195 to 210 degrees.) This speeds engine warm-up and reduces emissions. Thermostat failures are a common cause of overheating. Replacement thermostats must be the same temperature rating as the original. This is extremely important on late-model, computer-controlled engines that use the temperature reading from the coolant sensor to regulate the fuel mixture, ignition and other emission functions. Some vehicles have thermostats with a "jiggle pin" vent that allows trapped air to escape from the engine when the cooling system is refilled with coolant.


The water pump is a belt-driven pump that circulates coolant between the engine and radiator. The pump consists of an impeller mounted on a shaft inside a cast or stamped steel housing. Failure of the pump shaft seal or bearing can cause noise and coolant leaks. A defective water pump can be replaced with a new or remanufactured replacement pump.

The radiator is a large heat exchanger mounted in front of the engine. Airflow through the radiator provides cooling for the coolant that circulated through it. Most newer radiators are a "crossflow" design where the coolant flows from one end to the other. Older vehicles usually have "downflow" radiators where the coolant flows from the top to the bottom. Most newer radiators are aluminum, while many older radiators are copper/brass. Most radiators also contain a loop of pipe in the bottom tank or end tank for cooling automatic transmission fluid. A replacement radiator should have the same hose configuration (location and size) as the original and provide equivalent (or better) cooling. For towing applications and high-performance engines, a larger, thicker and/or more efficient radiator can be installed to improve cooling.


The radiator cap is a spring-loaded pressurized cap on the radiator that prevents coolant loss and increases the temperature at which the coolant boils. Pressure ratings vary from five to 15 psi. Caps should be replaced if they cannot hold their rated pressure. Replacement caps must have the correct pressure rating for the application to prevent overheating.

There are two basic types of cooling fan: belt-driven and electric (though a few vehicles have hydraulic fans that are driven by power steering fluid.) The fan increases airflow through the radiator for improved low-speed cooling. Belt-driven fans are mounted on the water pump pulley and may have a viscous clutch that allows the fan to slip at higher speeds when extra cooling isn’t needed. Electric fans may be mounted in front or behind the radiator and are powered through a relay. Some vehicles have a temperature switch to turn the fan on and off, while others use commands from the engine computer and coolant sensor to control the fan. Some vehicles have two electric fans, one of which may be used for the A/C condenser. A fan or fan clutch failure may cause the engine to overheat at low speed.


An engine-mounted coolant sensor monitors the temperature of the coolant. The sensor’s resistance changes as the temperature goes up. The sensor’s output voltage may be used to operate a temperature gauge or warning light, the cooling fan and various emission functions. It is also used by the engine computer to determine when the engine can go into "closed loop" operation (when the computer uses input from the oxygen sensor to regulate the fuel mixture.) A defective sensor can prevent the engine from going into closed loop, cause poor fuel economy and cause higher emissions.


The radiator and heater hoses are flexible rubber hoses that carry coolant between the engine, radiator and heater core. Most applications have an upper radiator hose, a lower radiator hose and at least two heater hoses. There may be an additional bypass hose or other connecting hoses. Some newer vehicles have "branched hoses" where one hose connects directly to another. Most original equipment hose is "molded" to shape, while some aftermarket replacement hose is "flex" hose that bends and can be used in a wider variety of applications. Hoses deteriorate with age and may leak, allowing loss of coolant and allowing engine overheating. Replacement hoses must be the same diameter and length as the original. Always recommend new clamps.


There are two basic types of belts: V-belts and flat (serpentine) belts. Belts are used to drive the water pump and other engine accessories. Belts deteriorate with age, and should be replaced if frayed, cracked, glazed or oil-soaked. Replacement belt length and width must be the same as the original. On vehicles with serpentine belts, the automatic tensioner 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.

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