Compression Gauge
A diagnostic tool that measures the pressure generated inside an engine cylinder during the compression stroke.
A compression gauge is a diagnostic tool consisting of a pressure dial, a flexible hose, and a threaded adapter that screws into a spark plug or glow plug hole. It is used to measure the pumping efficiency and mechanical seal of an engine's cylinders during a compression test.
To perform the test, all spark plugs are removed, the fuel and ignition systems are disabled, and the gauge is screwed into one cylinder at a time. The engine is cranked, and the gauge records the peak pressure (in psi) generated. A healthy engine should produce consistent pressure across all cylinders, typically within 10% of each other.
Low compression in one or more cylinders indicates a mechanical issue. If two adjacent cylinders have low compression, it often points to a blown head gasket between them. Low compression in a single cylinder can be caused by leaking valves, worn piston rings, or a cracked piston. Mechanics use a subsequent wet test (adding oil to the cylinder) or a leakdown test to pinpoint the cause.