To use a compression test to diagnose an engine, first disconnect the fuel pump, fuel injection fuses, the wire leading to the coil and spark plugs, and all spark plugs. Next, place the end of the compression gauge into one of the spark plug holes, inserting the threaded end first. Turn on the ignition and press on the throttle until the engine hits four revolutions. If the reading on the compression gauge is unstable, try taking it to 10 revolutions.
Repeat the process with all cylinders. Make sure to note the reading of each cylinder on its valve cover using chalk. It should take approximately 20 minutes to test the entire engine.
An engine in good form should achieve compression readings over 100 psi in each cylinder. If cylinders read below 100 psi, add a teaspoon of engine oil into the offending spark plug hole and try again. If the psi rises, the piston rings may be worn. If it does not, the valve may be malfunctioning.
Compression gauges are available for purchase at stores that specialize in vehicles. As of 2015, they retail for around $50, offering an inexpensive way to diagnose not only piston rings and valves, but whether or not head gaskets are in working order. Brands such as Actron/Professional include adapters that make the gauge compatible with spark plug threads ranging from 10 millimeters to 18 millimeters.