Originally Posted by SEAL

A leakdown tester is a device that connects to a compressor and the spark plug hole in the cylinder. In order to leak down test a cylinder, you connect the tester to a cylinder that is at TDC in the compression stroke. The tester applies pressure from the compressor to the cylinder. A gauge then measures how much air is flowing from the compressor to the cylinder (the less air, the better), and the amount of leakdown is meaured in percent. Lower is better. While this is happening, you can then find where the engine is leaking based on where the air is going. Air going out the valves indicates valves that are not sealing. Air going into the crankcase indicates bad rings. Air going into adjacent cylinders indicates a bad head gasket. And air going into water jackets indicates a bad head gasket, cracked cylinder and/or head, or a warped cylinder and/or head. A leakdown test is not necessarily better than a compression test in that it can detect problems where a compression test indicates no problems. But it can tell you exactly WHAT is bad when compression is low. For this reason, a leakdown test is generally used to complement a compression test rather than replacing it.