A standard compression test tells you if a cylinder has low compression, but a leak-down test tells you why. You set the cylinder to Top Dead Center (TDC) so both valves are closed, then screw in the tester hose. The gauge displays the percentage of air leaking past the seals. If the leak is high, listen for escaping air. Air escaping from the tailpipe indicates a bad exhaust valve. Air from the intake indicates an intake valve issue, and bubbles in the radiator point to a blown head gasket.
Mechanic Glossary
Cylinder Leak-Down Tester
A diagnostic tool that pumps compressed air into a spark plug hole to measure the percentage of cylinder pressure loss and find the leak source.
Frequently Asked Questions
A healthy engine should show less than 10 percent leak-down. Anything between 10 and 20 percent is acceptable, while over 30 percent indicates a serious engine problem.
At TDC, both the intake and exhaust valves are fully closed. If the piston is not at TDC, air will rush out through an open valve.
You can, but a warm engine is better. Piston rings expand and seal better when warm, so a cold test will show slightly higher leakage.