- Warm up the engine to normal operating temperature.
- Remove all spark plugs.
- Disconnect the high tension lead from the ignition coil.
- Fully open the throttle, either by operating the throttle linkage by hand or by having an assistant floor the accelerator pedal.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily ruined.
- Ask an assistant to depress the accelerator pedal fully on both carbureted and fuel injected vehicles. Then, while reading the compression gauge, ask the assistant to crank the engine two or three times in short bursts using the ignition switch.
- Read the compression gauge at the end of each series of cranks, and record the highest of these readings. Repeat this procedure for each of the engine's cylinders. Compare the highest reading of each cylinder to the compression pressure specification in the "Tune-Up Specifications" chart in . The specs in this chart are maximum values.
A cylinder's compression pressure is usually acceptable if it is not less than 80% of maximum. The difference between each cylinder should be no more than 12 and 1914 pounds.
- If a cylinder is unusually low, pour a tablespoon of clean engine oil into the cylinder through the spark plug hole and repeat the compression test. If the compression rises after adding the oil, it appears that the cylinder's piston rings or bore are damaged or worn. If the pressure remains low, the valves may not be seating properly (a valve job is needed), or the head gasket may be blown near that cylinder. If compression in any two adjacent cylinders is low and if the addition of oil does not help the compression, there is leakage past the head gasket. Oil and coolant water in the combustion chamber can result from this problem. There may be evidence of water droplets on the engine dipstick when a head gasket has blown.