What mechanism allows coolant to move from the expansion tank to the radiator?

The mechanism that allows coolant to move from the expansion tank to the radiator primarily involves the use of coolant circulation created by pressure differences, which typically utilizes the principles of vacuum. When the engine operates, the coolant heats up, and as it expands, it may rise to the expansion tank. When the system cools down, a vacuum can form in the cooling system, drawing coolant back from the expansion tank into the radiator.

This process ensures that the cooling system maintains the correct level of coolant, preventing air pockets that could lead to overheating. The expansion tank is designed to handle coolant overflow and facilitate the return flow back to the radiator, which is essential for maintaining optimal engine temperature and preventing damage.

Other options like the thermostat, pressure relief valve, and coolant pump play crucial roles in the cooling system but are not directly responsible for the flow of coolant from the expansion tank to the radiator. The thermostat regulates engine temperature by controlling coolant flow, the pressure relief valve helps in managing system pressure, and the coolant pump circulates coolant throughout the engine and radiator during operation. However, it is the vacuum mechanism that facilitates the movement specifically from the expansion tank to the radiator when conditions allow.