Cylinder Leakage Test
A cylinder leak test is usually performed after a cylinder pressure test has indicated a problem. The cylinder is placed on top dead center of the desired cylinder and the spark plug is removed on that cylinder only. This is because when air pressure is introduced into the cylinder the piston will travel downward. The resistance from the other spark plugs helps resist this downward movement.
Once the cylinder is at TDC, air pressure is introduced through a cylinder leak down gauge. These tools often have two gauges, one for inlet pressure and the other for leak down percentage. This second gauge indicates how much pressure the cylinder is actually losing. This loss is measured in percentage which is not to exceed 20 percent. Always check with manufacturer’s specifications.
Listen for air hissing through the throttle body or carburetor. This indicates a problem with the intake valve. It’s allowing compression to seep past the valve and its corresponding seat. Listen for air escaping through the tailpipe. This indicates a problem with a worn or burned exhaust valve. Air escaping through the oil fill cap indicates worn piston rings. Bubbles seen in the radiator neck indicates a leaking head gasket.