A leak down test uses compressed gas (usually air) feed into the spark plug port to test for cylinder leakage. Then engine is set at TDS on the compression stroke and warmed up for the test.
The leak down tester is a block consisting of 2 air pressure gauges with a precise port (orifice) between them. Compressed air inputs through an adjustable air pressure regulator to one gauge at 80 PSI and goes through the port to the other gauge then into the cylinder. The port provides a restriction to the airflow into the cylinder. The air output hose attaches to the cylinder the same way a compression tester does using the spark plug threads.
The difference between readings of the two gauges is an indication of leakage in the cylinder. If the two gauges read the same (80 psi) there would be zero or very little leakage. The lower the second gauges pressure, the more the leakage. Normally a reading of 60 psi or lower on the second gauge is cause for concern. For documentation readings are noted by the indications of both gauges, in this case it would be 80/60.
This test is more accurate and consistent then a compression test, does not require the engine be turned over but does require a source of compressed gas. The leak down gauge is more expensive to buy then a compression test gauge.
One thing to remember is the test inputs compressed air into the cylinder on the compression stroke. Thus unless the crank is locked it can turn as the piston is pushed down by the pressure.
