What opens a valve on the radiator cap?

A valve on the radiator cap is designed to maintain the system's pressure within an optimal range for effective cooling. Specifically, a vacuum plays a key role in this mechanism. When the engine cools down, the coolant contracts, creating a vacuum that pulls the valve open. This opening allows air to enter the cooling system, which prevents the formation of a vacuum that could otherwise lead to cavitation or damage to the system.

While pressure buildup is significant for keeping the system sealed and allowing it to operate at a higher temperature without boiling, it does not specifically trigger the opening of the valve. Similarly, temperature increase is associated with the functioning of the radiator cap's pressure relief mechanism, but again, it does not open the valve in question. Lastly, while coolant level is crucial for the overall health of the cooling system, it does not directly influence the operation of the radiator cap valve in terms of opening it. Therefore, understanding that a vacuum is what opens the valve clarifies its role in maintaining the cooling system's efficiency.