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how does a radiator cap work?

 
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It keeps the coolant under pressure, usually around 13 pounds, so it boils at a higher temperature. It has a spring that holds a metal and rubber cap against the opening of the radiator. When the coolant heats up and reaches a certain temperature, the coolant flows past the spring-loaded cap and into an overflow bottle. When the coolant cools down, it contracts, and is drawn back into the radiator. This system allows the radiator and the rest of the cooling system to operate with no air in it, because the presence of air can cause rust.

 
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Spring tension.















The cap has a calibrated spring that compresses at a chosen temperature. When the water reaches the temperature listed on the cap, the spring will compress allowing the water to pass to the over flow reservoir.