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Operation and Importance of the Automotive Cooling System

Purpose Of The Cooling System

The primary function of the engine cooling system is to control the amount of expansion of the internal parts when heated. All metals, along with other things, contract when cold, and expand when heated. This is a natural phenomenon.

In the case of an automobile engine, the block, cylinder heads and internal moving parts are affected by heat expansion. The parts, being of dissimilar metals and densities, tend to expand at different rates. All the internal parts are measured cold and in their expanded condition. They are sized to obtain complete expansion around 195-degrees F.

Method of Operation

The cooling system consists of a radiator to transfer heat to the atmosphere, a radiator cap, engine cooling fan and shroud, water pump, thermostat, hoses and heater core.

Although water cools rapidly, it creates rust and scales, evaporates quickly and boils at a low 212-degrees F. Antifreeze, when mixed in equal parts with distilled water, prevents rust, raises the boiling point, lubricates the water pump and resists freezing temperatures.

Hot coolant flows from the top of the radiator, cooling as it flows to the bottom of the radiator, then on to the water pump.

From the water pump, the coolant flows through cavities in the engine block and cylinder heads and up to the thermostat. The thermostat’s mission is to maintain the 195-degree temperature by varying the amount of coolant flow to the radiator.

When the engine is cold the thermostat remains closed to prevent circulation of the coolant. This allows the engine temperature to rise quickly to its optimum operating temperature. At 195-degrees, the thermostat has opened and circulates the coolant to maintain the 195-degree temp. The coolant flows through the thermostat housing back to the top of the radiator to relinquish its heat.

An engine driven fan is designed to operate when the temperature rises above 200-degrees. When the vehicle is in motion the air passing through the radiator cools the fan and begins to freewheel, lessening the drag on the engine.

Front wheel drive vehicles as well as some traditionally mounted engines use an electric fan. When the vehicle is at rest and the temperature rises, a coolant temperature sender signals the ECU, which in turn activates the fan relay and turns on the fan.

As the vehicle begins to move, the temperature drops and the ECU turns off the fan, lessening drag on the engine due to the additional load on the alternator.

Ways the Cooling System can be Compromised

  • Blocked or restricted air flow through the radiator
  • Plugged radiator from build up and contamination
  • Bad radiator cap not holding proper pressures
  • Low coolant level, improper coolant concentrations
  • Coolant leaks from radiator, hoses, water pump, heater core, radiator cap
  • Collapsed /collapsing hoses
  • Failed engine cooling fan and or fan clutch
  • Stuck thermostat – open or closed
  • Blown head gasket


Never open the radiator cap on a hot engine. Hot coolant or just the steam created by the over heated engine will blow out of the radiator, resulting in the potential for severe burns. Wait until the cap and radiator are cool enough to touch.

Never operate a vehicle that is overheating. When an indication of overheating is noticed, shut the engine down and allow it to cool. An overheated engine is likely to blow a head gasket, allowing coolant to enter the engine cylinders and crankcase.

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