Engine Compression Test

Charge the battery if the battery is not fully charged.
Disable the ignition system.
Disable the fuel injection system.
Remove all the spark plugs.
Turn the ignition to the ON position.
Depress the accelerator pedal to position the throttle plate wide open.
Start with the compression gauge at zero and crank the engine through four compression strokes.
Check the compression for each cylinder. Record the readings.
If a cylinder has low compression, inject approximately 1 tablespoon (15 ml) of engine oil into the combustion chamber through the spark plug hole. Recheck the compression and record the reading.
The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 100 psi (690 kPa). For example, if the highest pressure in any one cylinder is 150 psi (1 035 kPa), the lowest allowable pressure for any other cylinder would be 105 psi (725 kPa). (150 x 70% = 105) (1 035 x 70% = 725).

Normal - Compression builds up quickly and evenly to the specified compression for each cylinder.
Piston Rings Leaking - Compression is low on the first stroke. Then compression builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
Valves Leaking - Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.
If 2 adjacent cylinders have lower than normal compression, and injecting oil into the cylinders does not increase the compression, the cause may be a head gasket leaking between the cylinders.