Diagnosis And Testing - Radiator Cap
Remove cap from the pressurized coolant bottle. Be sure that sealing surfaces are clean. Moisten rubber O-ring seal with water and install the cap onto the commercially available cooling system tester.
Operate the tester pump and observe the gauge pointer at its highest point. The cap release pressure should be 1.4 bar (20.3 psi). The cap is satisfactory when the pressure holds steady. It is also good if it holds pressure within the 1.4 bar (20.3 psi) range for 30 seconds or more. If the pointer drops quickly, replace the cap.
Radiator pressure testing tools are very sensitive to small air leaks, which will not cause cooling system problems. 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 tool. Turn tool upside down and recheck pressure cap to confirm that cap needs replacement.