Believe it or not, your radiator cap is a key component of the cooling system. Other than keeping the coolant in the system, it serves some very important functions. For every pound of pressure listed on the cap, it raises the boiling point of the coolant 2º F. So if you have a 15-lb. cap, it raises the boiling point of the coolant mixture 30º F. This means that if you have a bad radiator cap, your cooling system won’t be as efficient as it could be.
The radiator cap serves another function, which is to control the flow of coolant during temperature changes in the cooling system. When the system is at operating temperature, the coolant expands, and it needs a place to go. A valve inside the radiator cap allows the heated coolant to exit the system into the overflow tank. When you shut the engine off and it begins to cool, another valve inside the radiator cap opens up to help draw that coolant back into the system. As the engine cools after you shut it off, the coolant inside the engine contracts and helps create a vacuum that draws the coolant from the overflow back into the system.
If either one of these valves fail, it can cause issues with the cooling system. A bad radiator cap can cause air to enter the system, or it can release coolant too early, causing the overflow tank to overflow and boil over.
In summary: Don’t forget about the radiator cap, as it is an important part of the cooling system. In fact, it’s often considered a maintenance item and is sometimes replaced during a service. I recommend using OE radiator caps whenever possible.
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