Cylinder Leak Down Test

Cylinder leakage areas of concern.

Run a cylinder leakage or leak down test after a cylinder compression test has verified a problem with the cylinder; a cylinder leakage test helps pinpoint the area of concern. It shows the percentage of leakage and helps identify the part causing the compression leak.

Cylinder Leak Down Tester

Cylinder leak down tester.

A cylinder leak down tester measures the amount of air pressure loss as a percentage. Typical testers indicate a problem when leakage exceeds 30-40%.

Safety Tip: 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.

Diagnosing Cylinder Leakage

Diagnosing cylinder leakage by sound.

If there is a significant leak, listen for air hissing through the throttle body, tailpipe, and valve cover's oil fill or PCV valve hole.

Leaking Intake Valve: A hissing sound from the throttle body indicates a problem with the intake valve. It allows compression to seep past the valve and its corresponding seat.

Leaking Exhaust Valve: Listen for air escaping through the tailpipe, indicating a problem with a worn or burned exhaust valve. The air/fuel charge helps cool the valve in multiport fuel injected engines. Therefore, the exhaust valves are more likely to burn with multiport injection.

Worn Piston Rings: Air escaping through the oil fill cap indicates worn piston rings. If all the piston rings fail the test in a high mileage engine, the engine is tired and needs new rings and bearings.

Leaking Head Gasket: Bubbles in the radiator neck indicate a leaking head gasket. The bubbles are from a compression leak between the water jacket and the cylinder. Engine overheating causes cylinder head warping and head gasket failure.