Engine Compression Test

Charge battery if battery is not fully charged.
Remove coil pack from each cylinder head.
Remove all of the spark plugs.
Disable ignition system, fuel injection, and open the throttle plates wide open, using a Tech 2 scan tool under the COMPRESSION TEST screen.
Start with compression gauge at zero.
Turn key to START position and release. The engine will automatically crank.
Make a compression check for each cylinder and record the readings.
If a cylinder has low compression, inject about .5 oz. (15 ml) of engine oil into the combustion chamber through the spark plug hole. Recheck 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 were 150 psi (1035 kPa) the lowest allowable pressure for any other cylinder would be 105 psi (725 kPa). 150 x 70 percent = 105 (1035 x 70 percent).
- Normal
  
  Compression builds up quickly and evenly to the specified compression for each cylinder.
- Piston Rings Leaking
  
  Compression is low on the first stroke. Compression then 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.
- Adjacent Cylinders
  
  If 2 adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase compression, the cause may be a head gasket leaking between the cylinders.