{"id":119181,"date":"2024-09-24T22:03:06","date_gmt":"2024-09-25T05:03:06","guid":{"rendered":"https:\/\/blogv2prod.wpenginepowered.com\/blog\/?p=119181"},"modified":"2025-01-11T09:41:02","modified_gmt":"2025-01-11T17:41:02","slug":"what-is-a-radiator-cap-functions-symptoms-replacement-and-more","status":"publish","type":"post","link":"https:\/\/www.carparts.com\/blog\/what-is-a-radiator-cap-functions-symptoms-replacement-and-more\/","title":{"rendered":"What Is a Radiator Cap? Functions, Symptoms, Replacement, and More"},"content":{"rendered":"\nReading Time: <\/span> 7<\/span> minutes<\/span><\/span>\n\n\n

A car’s cooling system<\/a> keeps the engine running at a safe temperature to prevent overheating. One component that aids in maintaining proper temperature is the radiator, which disperses a mix of antifreeze<\/a> and water to release the engine’s heat.<\/p>\n\n\n\n

The radiator has another component attached to it called the radiator cap. Despite its small size, the cap plays a big part in keeping the engine safe.<\/p>\n\n\n\n

Caution:<\/em><\/strong> Never, under any circumstances, remove the radiator cap from a hot cooling system to avoid serious injury.<\/em><\/p>\n\n\n\n

What Is a Radiator Cap?<\/h2>\n\n\n\n

Some cooling systems have a radiator<\/a> with no cap because the fill point is at the surge tank rather than on the radiator. If the radiator has a cap<\/a>, it will have a degas tank instead of a robust surge tank.<\/p>\n\n\n\n

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This is a coolant reservoir. This type won’t have a screw-on cap because it’s not designed to hold pressure. The coolant enters this bottle as it expands and then returns to the radiator as the engine cools and the radiator cap valve opens.
This is a surge tank or “degas” tank. This type will be designed to hold pressure and will have a screw-on pressure cap and is typically the fill point for the cooling system. |Image Source: Richard McCuistian<\/figcaption><\/figure>\n\n\n\n

This is a coolant reservoir. This type won’t have a screw-on cap because it’s not designed to hold pressure. The coolant enters this bottle as it expands and then returns to the radiator as the engine cools and the radiator cap valve opens.<\/p>\n\n\n\n

This is a surge tank or “degas” tank. This type will be designed to hold pressure and will have a screw-on pressure cap and is typically the fill point for the cooling system.
Older vehicles usually have the radiator cap on the radiator, which is fitted with a filler neck.<\/p>\n\n\n\n

\"automotive
This kind of cap has a spring-loaded valve that closes the cooling system vent, causing pressure to build up to the pressure setting of the cap. | Image Source: Richard McCuistian<\/figcaption><\/figure>\n\n\n\n
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The cap’s spring is calibrated to maintain a certain amount of pressure as the coolant expands. Usually that calibration is from 12 to 15 psi, but there are lower pressure calibrations available down to about 7 psi. | Image Source: Richard McCuistian<\/figcaption><\/figure>\n\n\n\n

Essentially, the cooling system is pressurized to raise the boiling temperature of the coolant. When the pressure rises above the cap’s threshold due to coolant expansion, the heavy spring is compressed, the gasket is lifted from its sealing seat in the radiator neck, and coolant travels to the degas tank until the pressure returns to the cap’s calibrated psi.<\/p>\n\n\n\n

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The small spring-loaded brass valve in the center of the cap is supposed to remain closed as long as there is pressure on the system but will open after the engine is shut off and the coolant contracts, which causes coolant to return from the degas tank to the radiator. If the valve’s spring fails, it will prevent the cap from holding pressure (see photo). | Image Source: Richard McCuistian<\/figcaption><\/figure>\n\n\n\n