Which component releases coolant when the system pressure becomes too great?

The component that releases coolant when the system pressure becomes too great is the radiator cap. Radiator caps are designed to maintain a specific pressure level within the cooling system. When the pressure exceeds the cap's rated limit, typically due to overheating or issues within the cooling system, the radiator cap opens and allows excess pressure to escape, which often results in coolant being released. This function helps prevent damage to the radiator, engine, and other components by controlling the pressure and allowing coolant to exit before the system can fail catastrophically. The release of coolant also allows for the expansion and contraction of fluid due to temperature changes, ensuring that the system operates effectively within safe pressure limits.

The water pump's primary role is to circulate the coolant throughout the engine and radiator, but it does not manage pressure directly. The coolant reservoir serves as a storage tank for excess coolant and allows for the collection of overflow but does not actively release coolant in response to pressure. The thermostat regulates the flow of coolant based on temperature, opening and closing to maintain optimal engine operating temperatures, but it does not act on pressure directly. Thus, the radiator cap is essential for maintaining system integrity and preventing damage through its pressure-relief function.