Radiator Pressure Cap Testing

Radiator Pressure Cap Testing

Special tools, testers and auxiliary items required

• Cooling System Tester (VAG 1274A)

• Cooling System Adapter (J-48840)








Dip the pressure cap (1) in water. Clean any deposits off the vent valve or its seat and install the cap to the cooling system adapter (J-48840). Using the cooling system tester (VAG 1274A) (2), work the plunger, bringing the pressure to 104 kPa (15 psi) on the gauge. If the pressure cap fails to hold a pressure of at least 97 kPa (14 psi), replace the pressure cap.


The cooling system tester (VAG 1274A) is very sensitive to small air leaks that will not a cause cooling system problem. A pressure cap that does not have a history of coolant loss should not be replaced just because it leaks slowly when tested with this tool. Add water to the tool. Turn the tool upside down and recheck the pressure cap to confirm that the cap is bad.

If the pressure cap tests properly while positioned on the cooling system adapter (J-48840) , but will not hold pressure or vacuum when positioned on the filler neck. Inspect the filler neck and cap top gasket for irregularities that may prevent the cap from sealing properly.