Which part in the cooling system controls the flow of coolant?

The thermostat plays a crucial role in the cooling system by controlling the flow of coolant. It is a temperature-sensitive valve that opens and closes based on the coolant's temperature. When the engine is cold, the thermostat remains closed, preventing coolant from circulating to the radiator. This allows the engine to reach its optimal operating temperature more quickly. Once the engine reaches this temperature, the thermostat opens and allows coolant to flow into the radiator, where it is cooled before being recirculated back to the engine. This regulation ensures that the engine operates at the correct temperature, enhancing efficiency and preventing overheating.

Other components like the heater core, radiator, and water pump are essential for the overall function of the cooling system; however, they do not control the flow of coolant in the same dynamic manner as the thermostat. The heater core provides heat to the cabin, the radiator dissipates heat from the coolant, and the water pump circulates the coolant through the system, but the thermostat is the critical component that opens and closes to regulate flow based on the engine's temperature needs.