What a leak-down test checks
Unlike a compression test, a leak-down test uses an external air supply to pressurize a cylinder when its valves should be closed. The tester reports how much air escapes under the chosen test conditions. This can help reveal that a cylinder is not sealing as expected and guide further inspection, but it does not automatically identify a failed part.
Results depend on the regulator, gauge design, calibration, air pressure, adapter, engine temperature and piston position. Two different testers may not produce directly comparable percentage readings. For that reason, use the same correctly calibrated equipment and method for every cylinder, and follow the manufacturer's limits rather than a universal online threshold.
Safety and test setup
Compressed air can rotate an engine when the crankshaft is not securely positioned, potentially moving belts, fans and other components. Follow the manufacturer's service procedure and the tester maker's instructions, secure the vehicle, keep hands and tools clear of moving parts, and use suitable eye protection. If you're unfamiliar with the procedure, have a qualified technician conduct the test.
What the sound of escaping air can suggest
The location where air exits can be a useful observation, although a sound is not a conclusive diagnosis. Ask a qualified technician to correlate the escape path with the valve-train design and repeatable measurements.
| Where air appears to escape | Potential area to inspect |
|---|---|
| Intake tract | Intake valve sealing, or incorrect test position |
| Exhaust outlet | Exhaust valve sealing |
| Crankcase / oil-filler opening | Ring or cylinder-wall leakage; some leakage may be normal |
| Cooling system | Possible head gasket, cylinder head or block path—requires confirmation |
Why a percentage isn't a universal grade
A displayed 10%, 15% or 20% does not translate cleanly into one engine-health grade. Instruments have different reference settings, and some cylinders naturally leak more than others. An identical percentage on a different tester may not represent identical physical leakage.
A useful worksheet records the tester and calibration method, pressure setting, engine temperature, cylinder position and any audible leak location. Compare each cylinder under the same conditions, especially when investigating one suspected cylinder.
How to combine leak-down and compression measurements
A dry compression reading is affected by dynamic cranking conditions; leak-down assesses sealing with the piston at a controlled position. Low cranking compression and unusually high leakage in one cylinder can support further investigation, but do not determine the cause without other evidence. Disagreement between tests can be a reason to recheck setup, timing or other conditions.
Use our free compression-and-leak-down worksheet to record both kinds of readings. It provides an optional place for leak-down percentages but deliberately does not label any single percentage a definite pass or fail.