What is inside the car expansion tank cap?
3 Answers
The car expansion tank contains coolant. Introduction to coolant: Coolant, whose full name should be antifreeze coolant, refers to a coolant with antifreeze function. Antifreeze can prevent the coolant from freezing and expanding the radiator or damaging the engine cylinder block when the car is parked in cold seasons. Antifreeze is not only used in winter; it should be used all year round. Function of the expansion tank: The function of the car expansion tank is to ensure that the radiator is filled with coolant, providing space for the coolant to expand when heated and compensate for the volume when cooled. It also maintains a certain reserve of water to avoid consumption and water shortage.
The car expansion tank cap hides a small mechanism, which is essentially a spring-loaded pressure valve. The spring's tension determines the entire cooling system's pressure, usually maintained around 1.0-1.5 kg. The cap has a rubber seal pressing against the tank opening, with a movable valve underneath. When your car heats up, the expanding coolant pushes the valve open to release pressure; as it cools down, it pulls the valve open to replenish coolant. This opening and closing action maintains pressure, keeping the coolant's boiling point higher. If the cap's seal cracks or the spring weakens, the car is prone to overheating or coolant leaking everywhere.
When I last changed the coolant, I studied this cap—it's essentially an automatic pressure regulator. Inside, there's a two-stage valve composed of a metal spring and a rubber gasket. When the car is running and the coolant temperature rises, the cap traps steam in the system to increase the boiling point. Then, when the car is parked and cools down, it automatically draws coolant back from the expansion tank. This design is particularly clever, eliminating the hassle of manual bleeding. However, if the rubber in the cap degrades over time, the pressure settings can deviate, leading to radiator leaks. It's recommended to check the cap's seal integrity roughly every 60,000 kilometers.
