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The Cooling System

The cooling system found in your vehicle is designed to regulate the temperature at which your engine operates. Acceptable ranges of temperature depend on the type of engine and manufacturer specifications for that engine.

The cooling system has several basic components. All of these pieces need to work properly for the cooling system to regulate the engine temperature.

The basic components of a cooling system

  • Coolant or antifreeze
  • Radiator
  • Radiator cap
  • Cooling fan
  • Hoses
  • Water passages in the engine
  • Water pump
  • Thermostat
  • Engine coolant temperature sending unit and gauge
  • Heater core

These components circulate the engine coolant and allow the engine to maintain a proper operating temperature. This list is not all inclusive as manufacturers now add multiple sensors and other controls to the cooling system that help control other aspects of engine and heater operation. These additions will not be discussed to maintain clarity.

Coolant or antifreeze

Coolant or antifreeze is the fluid used to cool the engine. Water cannot be used alone as it will freeze and cause rust. Coolant is mixed with water to protect the engine to a desired temperature and deter rust. Coolant comes in many types of colors. The colors may indicate the type of coolant and service interval. Check with the manufacturer of the vehicle for specific details about service to the cooling system.


The radiator allows the hot coolant from the engine to cool down.  It is located at the front of the engine compartment behind the grille because it uses air flow to remove the heat from the coolant. It is rectangular in shape and has many fins between the water passages. The fins create surface area that allows the heat to dissipate quickly. The coolant flows from top to bottom in the radiator. Radiators usually have a drain on the bottom to allow the coolant to be drained when the cooling system requires service.

Radiator Cap

The radiator cap increases the boiling temperature of the cooling system. Radiator caps are rated to a certain psi (pounds per square inch). As coolant heats up, the pressure in the cooling system increases. The radiator cap is designed to contain this pressure in the system. When the coolant reaches a temperature that is greater than normal, excessive pressure is created in the system. When this pressure exceeds the rating of the cap, coolant boils out of the system and creates an overheating condition.

Cooling Fan

The cooling fan helps regulate the air flow across the radiator. It provides maximum air flow across the radiator at different operating conditions. There are several types of cooling fans. Electric fans can be mounted in front of or behind the radiator and are controlled electronically. They can work at several settings, high, medium, low or off as required. Some engines use mechanical fans that are attached to the water pump on the engine. Mechanical fans only work when the engine is running. Some mechanical fans incorporate a fan clutch. The clutch is fluid filled and is what regulates the speed of a mechanical fan. Some fan clutches are electronically controlled and monitored. To a lesser extent, some cooling fans are controlled by a hydraulic pump.


Cooling system hoses connect cooling system components to one another. They are generally made of rubber or silicone rubber. The hoses allow coolant to travel to different areas of the cooling system. Major cooling system hoses are the upper and lower radiator hoses, and supply and return hoses to the heater core. Coolant flows through these hoses to help regulate the temperature of the engine and to provide heat to the heater core of the heating system. Some vehicles have secondary heater hoses for auxiliary heating or peripheral systems. For example, auxiliary cooling system hoses can be used to heat the throttle body or PCV valve.

Water passages in the engine

Engine blocks are cast as one piece of metal. When the engine is cast, there are passages that are designed around the cylinders to aid in the casting process that ultimately  allow coolant help remove heat from the engine as the water flows through them towards the water pump. Water passages also run through the cylinder heads as well. Sometimes the coolant passages are visible as raised areas on the outside of the engine block. These passages are sealed by a round plug. This round plug is referred to as a freeze plug. The name originates from the idea that if the coolant passage was full of water and the water froze; the plug would be pushed out by the expanding water as it turned to ice. Some engines have a freeze plug with an AC cord and plug on it. These types of plugs are engine block heaters.

Water pump

The water pump is what circulates coolant throughout the engine. It consists of an impeller that is typically driven by a belt. The pump is generally bolted to the front center of the engine. On some engines the water pump may be mounted slightly off center and behind the timing belt cover. These pumps may be driven by the timing belt instead of an auxiliary or serpentine belt.


The thermostat regulates the temperature of the engine. It is made of a bimetal spring that opens when a certain temperature is reached. You may find the opening temperature stamped on some thermostats. The thermostat is closed until it reaches the opening temperature. This limits the flow of coolant to the radiator. By limiting the flow of coolant, the antifreeze will heat up quicker which will allow the engine to reach operating temperature faster. When the coolant reaches the temperature the thermostat is rated to, the thermostat opens and allows coolant to flow from the engine to the radiator.

Engine coolant temperature sending unit and gauge

The temperature sending unit is merely a thermistor. It is located in line of the cooling system and contacts the coolant inside the engine. The temperature of the coolant is detected by the sending unit and is transferred to the gauge on the dash. Do not confuse the engine coolant temperature sending unit with the engine coolant temperature sensor. Also known as the ECT, this sensor provides information to the engine management module about the engine coolant temperature to regulate engine operating conditions.

Heater core

The heater core provides heat in the heater system. It looks like a radiator and is about the size of an iPad. The heater core is typically located inside the heater case behind the dash panel. Coolant flows through the heater core. When the climate control system is set to heat and the fan is switched on, air blows across the heater core and into the cabin providing warm air. Some vehicles have cold engine lock out servos. These servos do not allow coolant to flow through the heater core until the coolant has reached a desired temperature. They are typically found inline of the heater hoses.

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