A diagnostic test that measures cylinder air leakage by introducing pressurized air. Identifies the location of compression problems (rings, valves, head gasket).
Leakdown test is a diagnostic procedure that introduces pressurized air into a cylinder and measures how much air leaks out. By listening or watching where the air escapes, the technician can identify the specific source of compression problems: leaking past the piston rings (air heard at the oil filler or exhaust), leaking through the valves (air heard at the intake or exhaust), or leaking through the head gasket (air heard at the radiator).
The test is more diagnostic than a compression test alone. While compression testing reveals which cylinders have problems, leakdown testing reveals where the problems are. The combination of compression and leakdown testing is the standard approach for diagnosing engine compression problems and provides clear guidance for repair decisions.
The test is performed with the engine cold or moderately warm. Each cylinder is rotated to TDC (Top Dead Center) on the compression stroke, the spark plug is removed, the leakdown tester is connected, and pressurized air is introduced. The percentage of leakage is read on the gauge; healthy cylinders typically show 5-10% leakage. Higher leakage indicates problems; the source is identified by listening for air at various engine locations. The cost is typically similar to compression testing ($75-$200 in shop labor).