If your engine runs bad, misfires on one or two cylinders, has rough idle, low compression, or may be losing coolant, you can do a compression test. For a stock engine, compression pressure at cranking speed should be in the 120-145 psi range (as high as 175 for flat top pistons). Compression should be the same an all cylinders within about +/-10%.

Compression testers are pretty cheap, some may be under $10. For an engine with clear access to the spark plug ports the easiest tester to use is one with the rubber end fitting. Just hold it tight into the spark plug port and crank the engine over several turns until the gauge stabilizes at maximum indication. Press the Schrader valve in the tip end to release the pressure.

For engines where the spark plug ports are harder the reach, a tester with hose and threaded fitting can work better. Screw this into the spark plug port and use in same manner. These commonly have two threads with O-ring seals to work in two different size threaded ports. For more information on compression problems see article BE-110.