Defective Radiator Pressure Cap

The radiator pressure cap is designed to maintain a sealed cooling system while relieving pressure when the engine is running hot. As the engine heats up and the coolant expands, the cap will allow the coolant to fill the overflow tank. When the engine cools down, it will relax, allowing coolant to be drawn from the tank into the system. If the cap is defective or faulty, it can prevent the coolant from overflowing to the overflow tank or prevent its return to the radiator.