For 1990-2009 cars

The pressure valve in the radiator cap will open and coolant will flow into the reservoir when the coolant expands (when the temperature rises). A vacuum is created in the radiator as the system temperature drops. This opens the vacuum valve in the cap and pulls coolant from the reservoir back into the radiator. This system maintains a constant engine coolant level (ECL) in the radiator for cooling the engine.

Maintain the ECL between the FULL and LOW marks on the coolant reservoir. Add coolant only through the coolant reservoir. Never add coolant directly into the radiator.