Cylinder Leakdown Test
A diagnostic test that measures the percentage of compressed air leaking from an engine cylinder to locate internal mechanical wear.
A cylinder leakdown test is a diagnostic procedure used to evaluate the internal sealing efficiency of an engine's combustion chambers. During the test, a technician inserts pressurized shop air into a cylinder that is set at Top Dead Center (TDC) on its compression stroke (when all valves are closed). Using a dual-gauge tester, they compare the incoming pressure to the pressure held within the cylinder, calculating the percentage of lost air.
A healthy engine typically exhibits less than 10% to 15% leakdown. Excessive leakdown (above 20%) indicates a mechanical seal failure. The key advantage of a leakdown test over a standard compression test is that it helps locate the exact source of the leak: air escaping through the tailpipe points to a leaking exhaust valve; air from the throttle body points to an intake valve leak; air escaping through the oil dipstick tube indicates worn piston rings; and bubbles in the radiator indicate a failed head gasket.
Performing a cylinder leakdown test requires specialized gauges, an air compressor, and the mechanical knowledge to set the engine cylinders to exact compression TDC. It is commonly recommended when an engine is misfiring, burning excessive oil, or showing low compression in one or more cylinders during a pre-purchase inspection.