1. A collapsed hose. This would be caused from a bad vacuum valve. The vacuum valve allows coolant to be drawn back into the radiator during cool down. If this is not working, the negative pressure created in the radiator during cool down could collapse your hose.
  2. Reservoir is overflowing. A radiator cap has a pressure release valve that allows water to flow into the reservoir tank. If the spring that operates this valve is weakened it will allow coolant to flow into the reservoir tank prematurely. This excess volume could result in the reservoir overflowing.
  3. Overheating in the engine. If the radiator cap is not properly sealing the system, it will allow air in. Air in the system makes it impossible to sustain consistent temperature. As a result, the system will overheat.

The radiator cap is the least expensive and most overlooked part in your cooling system. If you experience any of the issues we just discussed, the first thing to do is get a new cap. Also, whenever you replace a radiator you should always get a new cap.