Cylinder Leakage Test

Performed a cylinder leakage or leak-down test after a cylinder balance, and a cylinder pressure test has verified a problem with the cylinder. It shows the percentage of leakage and pinpoints the part causing the compression leak. Remove the spark plug and place the problem cylinder at (TDC) top dead center of the compression stroke. Remove the spark plug on that cylinder only.

Be careful, when introducing air pressure into the cylinder, the piston can suddenly travel downward. The resistance from the other spark plugs helps resist this downward movement.

Cylinder leakdown tester

Once the piston is at TDC of the compression stroke, air pressure is introduced through a cylinder leak down gauge. These tools have two gauges, one for inlet pressure and the other for leak down percentage. This second gauge indicates how much pressure the cylinder is losing. This loss is not to exceed 10-20 percent. Always check with the manufacturer’s specifications.

Diagnosing Cylinder Leakage

Diagnosing cylinder leakage by sound.

Listen for air hissing through the throttle body or carburetor. This sound 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, indicating 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 indicate a leaking head gasket.