Compression Test
A test that measures the pressure built up within each engine cylinder during cranking to assess the health of valves, pistons, and rings.
An engine compression test is a fundamental diagnostic test that measures the maximum air pressure (measured in PSI or Bar) generated within a cylinder during its compression stroke. The test is performed by removing the spark plugs, connecting a compression gauge to a cylinder, disabling the fuel and ignition systems, and cranking the engine with the throttle wide open.
Each cylinder is tested individually, and the results are compared to the manufacturer's specifications. More importantly, the readings must be consistent across all cylinders; a difference of more than 10% to 15% between the highest and lowest cylinder indicates a problem. Low compression in a single cylinder can cause a severe misfire, rough idling, and poor performance.
If low compression is detected, a technician can perform a 'wet' compression test by adding a teaspoon of engine oil to the low cylinder and repeating the test. If the compression rises significantly, the oil is sealing worn piston rings, indicating bottom-end wear. If the compression remains low, the issue is likely a leaking valve or a blown head gasket, requiring top-end engine repair.