One of the most overlooked causes of overheating in a vehicle is the radiator cap. The radiator cap holds pressure on the cooling system to raise the boiling point of the coolant. Every one pound of pressure present in the system raises the boiling point of the coolant three degrees. Typical late model radiator caps raise the boiling point of the coolant to around 126 degrees C F. Normal operating temperatures of 98.8 to 104 degrees C can cause overheating without the pressure supplied by the cap.
Allow the system to cool and remove the cap. Inspect the seal for damage. Heat cycling of the seal hardens the rubber, and split seals are a common source of leaks. Damaged seals require replacement of the radiator cap. Other damage, such as bent or broken recovery valves (the recovery valve is a disc shaped piece in the centre of the cap.) and rusted springs require replacement of the cap.
Install the cap onto the radiator cap adaptor supplied with the tester set. This adaptor looks like a radiator filler neck on both ends. Install the cap on one end, and attach the other end of the adaptor to the pressure tester. There are several different sizes and shapes of radiator cap, and several sizes and shapes of adaptors. Select the adaptor that is the same shape as the radiator filler neck on your specific vehicle.
Pump the pressure tester to the pressure stamped on the radiator cap. If the pressure releases the pressure before reaching the correct pressure, or the cap does not hold pressure, the cap is faulty. Remove the cap from the adaptor, and reinstall it on the adaptor. Repeat the test to verify that the cap is faulty.