A compression tester measures how much pressure an engine cylinder can build when cranking. Low pressure leads to rough idling, misfires, and a lack of power. The tester consists of a dial gauge, a flexible hose, and threaded adapters that screw into the spark plug hole. To run the test, you disable the ignition and fuel systems, remove all the spark plugs, and prop the throttle wide open. Cranking the engine five to six times pumps air into the gauge, showing the peak pressure in pounds per square inch (PSI).
Healthy gasoline engines usually show between 125 and 180 PSI per cylinder. The exact number is less important than how the cylinders compare to one another. The lowest cylinder should be within 10 to 15 percent of the highest. If one cylinder reads extremely low, squirt a tablespoon of engine oil into the spark plug hole and test it again. If the pressure rises, the piston rings are worn. If the pressure stays low, you have a leaking valve or a blown head gasket.