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Leakdown test

Car culture term

What is Leakdown test?

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).

Related terms

Compression testHead gasketValve adjustment

Frequently asked questions

How is leakdown different from compression test?
Compression test measures peak cylinder pressure during the compression stroke (an indicator of overall cylinder seal). Leakdown test measures how much air leaks from a pressurized cylinder (revealing the location of leaks). Compression alone reveals which cylinders have problems; leakdown reveals where the problems are. Both tests are typically used together for complete diagnosis.
What is normal leakdown?
Less than 10% leakage is typically healthy. 10-20% leakage indicates wear (often acceptable for street use). Above 20% indicates significant problems. Above 30-40% requires repair. The exact acceptable range varies by engine and use; track use cars often require tighter leakdown specifications.
How do I know which problem is causing leakage?
By listening for air at various engine locations. Air at the oil filler or dipstick indicates piston ring or cylinder wall problems. Air at the intake manifold indicates intake valve sealing problems. Air at the exhaust indicates exhaust valve sealing problems. Air at the radiator (with cap removed) indicates head gasket problems. The test method is direct and revealing.
Can I do a leakdown test myself?
Yes, with the right tools. A leakdown tester (about $60-$200) and an air compressor are required. The test is moderately complex; the engine must be at TDC for each cylinder, and the pressure must be carefully controlled. DIY testing is possible with care. Many DIY mechanics use this test for diagnostic purposes.
When should I do a leakdown test?
When troubleshooting engine compression issues. After modifications or rebuilds to verify proper assembly. Before purchasing a used vehicle. As part of routine pre-track-day inspection on track-focused cars. The test is moderately invasive but quick (15-30 minutes per cylinder typically). The information value usually justifies the time investment.
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