What is Coolant?

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  • Written By: J.M. Densing
  • Edited By: R. Halprin
  • Last Modified Date: 12 September 2019
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Coolant is a fluid that flows through a vehicle's engine block to keep moving parts from becoming too hot. It transfers heat from the engine to the radiator, thus preventing the engine from overheating. It's made out of a substance called glycol along with some other additives that prevent rust and is customarily mixed with water when in use. Coolant typically contains properties that prevent the fluid from freezing and is also commonly known as antifreeze. It's important to check levels of this vital fluid on a regular basis, monitor the system for leaks, and change it as recommended by the vehicle manufacturer.

When an engine runs, it creates a lot of heat from the combustion of fuel in the cylinders to provide power. This heat can be very harmful if it's allowed to build up and can actually destroy the engine. The friction created by the moving parts when there are high levels of heat can cause parts to change shape or to seize, which means that they melt and stop moving completely. Engine coolant is a vital fluid for a vehicle that prevents overheating by absorbing excess heat and carrying it away to the radiator. By performing this function, the coolant keeps the engine operating at a safe temperature.


Coolant consists of either propylene or ethylene glycol as its primary ingredient, together with substances called rust inhibitors which prevent engine and cooling system parts from rusting. The substance is usually mixed with water in a vehicle according to the manufacturer's instructions. The ratio of coolant to water is commonly 50:50, but sometimes other proportions are called for, particularly if the vehicle operates in severe conditions. In addition to keeping the engine from overheating, glycol also prevents the fluid from freezing in the winter so that the vehicle operates properly in severe weather.

Because the coolant is so important to engine operation, it should be maintained properly. The coolant level should be checked on a regular basis; this is simple to accomplish by looking at the side of the overflow tank which shows the appropriate levels when the engine is hot or cold. If the fluid level is below the required amount, more should be added to return it to the necessary level; the system should also be checked for leaks. Antifreeze should be handled with extreme caution since both fluid types can be poisonous to children and animals. To keep the system operating as designed, the fluid also needs to be replaced on a periodic basis as recommended by the vehicle manufacturer.


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