Engine Compression Test

A compression pressure test of the engine cylinders determines the condition of the rings, the valves and the head gasket.

1. Run the engine until it reaches normal operating temperature. The battery must be at or near full charge.
2. Turn the engine OFF.
NOTE: Remove the Powertrain Control Module (PCM) fuse and ignition fuse from the IP fuse block.
3. Disable the ignition.
4. Disable the fuel systems.
5. Remove the spark plugs from all cylinders.
6. Remove the air duct from the throttle body.
7. Block the throttle plate in open position.
8. Measure the engine compression, using the following procedure:
   • Thread the Spark Plug Port Adapter into the spark plug hole.
   • Thread the compression gauge into the Spark Plug Port Adapter.
   • Have an assistant crank the engine through, at least 4 compression strokes, in the testing cylinder.
   • Check and record the readings on the gauge at each stroke.
   • Remove the compression gauge from the Spark Plug Port Adapter.
   • Remove Spark Plug Port Adapter from the spark plug hole.
   • Repeat the compression test for each cylinder.
9. Record the compression readings from all of the cylinders. The lowest reading should not be less than 70 percent of the highest reading. No cylinder reading should be less than 140 psi (965 kPa).
10. The following are examples of measurement problems:
    • When the compression measurement is normal, the compression builds up quickly and evenly to specified compression on each cylinder.
    • When low compression is caused by the piston rings, compression is low on the first stroke and tends to build up on the following strokes, but does not reach normal. Compression improves considerably with the addition of oil. Use about 3 squirts of oil from a plunger-type oiler.
    • When low compression is caused by the valves, the measurement is low on the first stroke and does not build up even with the addition of oil.
    • Leaking head gaskets will provide the same results as worn valves but engine coolant may be identified in the crankcase. In addition, a leaking head gasket will give low readings on paired cylinders.
11. Remove the block from the throttle plate.
12. Install the air duct to the throttle body.
13. Install the spark plugs. Tighten to 13-17 ft. lbs. (17-23 N.m).
14. Install the Powertrain Control Module (PCM) fuse from IP fuse block.
15. Install the ignition fuse to the IP fuse block.