Engine Compression Test

Special Tools

• EN-51110  Compression Tester Adapter
• EN-26999  Compression Gauge Adapter

1. Ensure that the vehicle's batteries are in good condition, and fully charged.
2. Operate the vehicle until the engine is at normal operating temperature.
3. Shut off engine.
4. Remove all of the fuel injectors. All of the fuel injectors must be removed from the engine during each cylinder test to obtain valid test results.
5. Prior to taking a compression reading, verify the cranking speed is greater than 140 RPM. If the cranking speed is below 140 RPM, repair the slow cranking speed condition before continuing with the compression test. Refer to Engine Cranks Slowly .
6. Install the EN-51110  adapter in the fuel injector hole for the cylinder that is being checked.
Fig 1: EN-26999 Gauge And EN-51110 Adapter
Courtesy of GENERAL MOTORS COMPANY
CAUTION:

Do not add oil to any cylinder during a compression test as extensive engine damage may result.

7. Connect the EN-26999  gauge to the EN-51110  adapter.
CAUTION:

Do not crank the engine using the ignition switch, as the fuel injectors and the glow plug circuits will be energized and engine damage may result.

8. Using a remote start switch, crank the engine for 6 compression strokes, puffs, for the cylinder being tested.
9. Observe the EN-26999  gauge and note the reading as the compression test is being performed. A normal cylinder reading will be indicated if compression builds up quickly and evenly to the specified level. An abnormal reading will be indicated if compression is low on the first compression stroke, starts increasing on the following compression strokes but does not reach the specified level.
10. Record the compression reading for the cylinder just tested.
11. Disconnect the EN-26999  gauge from the EN-51110  adapter and remove adapter from the fuel injector bore.
12. Repeat steps 8 through 13 for all remaining cylinders. All 4 cylinders must be tested to obtain valid test results. Record the readings.
13. The minimum compression in any one cylinder is 2069 kPa (300 psi). There should not be more than 345 kPa (50 psi) difference between a suspect cylinder and the average compression of all 4 cylinders.
    NOTE:

    Cylinder to cylinder variation may be at the high end of the specification on engines with less than 20, 000 km (12, 427 miles).

    • 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 builds up with the following strokes but does not reach normal.
    • Valves Leaking - Compression is low on the first stroke. Compression usually does not build up on the following strokes.