Test how well your engine cylinder holds compressed air. Often used after getting a low reading from a compression tester, these leak testers show the percentage of the air leaking out of your cylinder. Remove all spark plugs from your engine and rotate it so the cylinder being tested is at the top. Make sure the cylinder is top dead center on its stroke, with both valves closed. Connect one end of these testers to the spark plug hole, and the other to a compressed air source. Fill the cylinder to its specified air pressure, and the gauge on these testers will show how much air is leaking out.