A wax-filled thermostat controls the engine operating temperature during its warm-up. In the first minutes after starting a cold engine, the thermostat remains closed, thereby preventing the coolant from circulating through the radiator. When the engine temperature reaches the normal operating value, the thermostat valve opens, connecting the radiator to the cooling circuit, ensuring maximum intensity of heat removal from the working fluid (coolant).
The cooling system has a hermetically sealed design and is tightly closed by a radiator cap capable of withstanding a certain excess pressure, which increases the boiling point of the coolant and, accordingly, the efficiency of heat removal through the radiator. When the internal pressure in the system exceeds a certain value, the spring-loaded plate of the safety valve mounted in the radiator cap rises above its seat, ensuring the flow of excess coolant through the connecting (overflow) tube into the expansion tank. As the system cools down, the liquid automatically returns from the tank to the radiator.
Adding coolant to the system is done through the neck of the expansion tank, which also acts as a receiver, accumulating excess liquid displaced from the radiator.
Due to the listed design features, such a cooling system is called closed, since any functional losses of coolant are excluded.
[Refer to the original source on the website: HONDABOOK]