Tools Required

J 38722 Compression Tester

Notice: The 3100 SFI V6 Engine L82 (VIN M) engine has aluminum cylinder heads. Allow the engine to cool before removing the spark plugs, or the engine may be damaged.

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.
3. Disable the ignition.
4. Disable the fuel systems.

Important: Remove the ignition fuse and both of the injector fuses from the fuse box and/or underhood electrical center. Refer to Electrical Diagnosis.

5. Remove the spark plugs from all cylinders. Refer to Ignition System.
6. Remove the air duct from the throttle body. Refer to Engine Controls.
7. Block the throttle plate in open position.
8. Measure the engine compression, using the following procedure:

8.1.	Install J 38722 firmly to the spark plug hole.

8.2.	Crank the engine through at least ten compression strokes (or until the highest reading is achieved) in the cylinder being tested and check the readings on J 38722 at each stroke. Record the results.

8.3.	Disconnect J 38722 .

8.4.	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% of the highest reading.

•	No cylinder reading should be less than 689 kPa (100 psi).

10. When the compression measurement is normal, the compression builds up quickly and evenly to the specified compression on each cylinder.
11. The following are problem areas:

•	If there is a problem with the piston ring, 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 approximately three squirts of oil from a plunger-type oiler.

•	If there is a problem with the valve, compression is low on the first stroke, and it does not tend to build up in following strokes. Compression does not improve much with the addition of oil. Use approximately three squirts from a plunger-type oiler.

•	Leaking head gaskets give nearly the same results as valve problems, but may be identified by engine coolant in the crankcase. Head gasket leakage between two cylinders will give low readings on both cylinders.

12. Remove the block from the throttle plate.
13. Install the air duct to the throttle body. Refer to Engine Controls.
14. Install the spark plugs. Refer to Ignition System.
15. Install the ignition fuse and both fuel injector fuses to the fuse box and/or underhood electrical center. Refer to Electrical Diagnosis.