Engine Compression Test

Tools Required 

SA9127E  Gage Bar Set. See Special Tools .

If there is a lack of power, excessive oil consumption, or poor fuel economy, etc., measure the compression pressure.

Fig 1: Removing & Installing Ignition Control Modules
Courtesy of GENERAL MOTORS CORP.
CAUTION: Compression checks must be performed with the vehicle in park or neutral and the parking brake applied.
1. Start the engine and allow it to reach normal operating temperature.
2. Shut the engine OFF and disconnect the ignition module wiring harness plugs.
3. Remove the ignition modules. Refer to Ignition Control Module Replacement - Bank 1 and Ignition Control Module Replacement - Bank 2 in Engine Controls - 3.0L
4. Remove all 6 spark plugs.
5. Insert the SA9127E  into the spark plug hole. See Special Tools .
6. Fully open the throttle.
IMPORTANT: Always use a fully charged battery to obtain engine revolutions of a 250 RPM minimum. Do not crank the engine for more than 15 seconds.
7. While cranking the engine, measure the compression. The cylinder should puff or the compression gage needle should bounce a minimum of 10 times.
IMPORTANT: All measurements must be completed in as short a time as possible. Check low cylinders and make sure a minimum of 10 puffs are obtained prior to reading the gage.
8. Repeat the previous steps for each cylinder.
   Fig 2: 3.0L RPO Code L81 V6 Engine Assembly
   Courtesy of GENERAL MOTORS CORP.
   IMPORTANT: No compression in 2 or more cylinders is an indication of damaged timing chain sprockets, camshafts, and crankshaft key or pins.
   IMPORTANT: The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi).
   • Normal compression: Builds up quickly and evenly to specification in each cylinder.
   • Low compression: Damaged piston, rings, valves, or head gasket.
   • High compression: Carbon on pistons/combustion chamber.
Fig 3: Adding Engine Oil To Cylinder Through Spark Plug Hole
Courtesy of GENERAL MOTORS CORP.
IMPORTANT: A cylinder leakage test can be performed to further determine the location of the compression leakage if one or more cylinders are extremely low.
9. If the cylinder compression in one or more cylinders is low, pour a small amount (1 teaspoon) of engine oil into the cylinder through the spark plug hole.
Fig 4: Placing Shop Towel Over Spark Plug Holes
Courtesy of GENERAL MOTORS CORP.
10. Place a shop towel over the plug holes to capture any oil mist/spray.
11. Crank the engine over for a few seconds without the compression gage or spark plugs installed.
12. Repeat previous steps 4-7 and measure the compression.
Fig 5: Checking Piston Rings & Piston Ring Land Wear
Courtesy of GENERAL MOTORS CORP.
13. If adding oil increases the compression readings the piston rings are leaking.

    Compression is low on the first stroke (first gage puff) and tends to build up on following strokes, but does not reach normal compression pressure. The compression Improves considerably with oil. Inspect for worn or broken rings and piston ring land wear (pound out).

Fig 6: Removing Fixed Timing Chain Guide Access Plug
Courtesy of GENERAL MOTORS CORP.
14. If adding oil does not increase the compression:
    • Valves or head gasket leaking: Compression is low on the first stroke. Compression does not tend to build up on the following strokes. Compression does not improve with adding oil to the cylinder.
    Fig 7: Checking Head Gasket If Two Adjacent Cylinders Have Lower Than Normal Compression
    Courtesy of GENERAL MOTORS CORP.
    • Head gasket: Two adjacent cylinders have lower than normal compression and adding oil to the cylinder does not improve compression.
Fig 8: Removing & Installing Ignition Control Modules
Courtesy of GENERAL MOTORS CORP.
NOTE: Refer to Fastener Notice in Cautions and Notices.
15. Remove the compression tester and install the spark plugs.

    Tighten:  Tighten the spark plugs to 25 N.m (18 lb ft).

16. Install the ignition module.

    Tighten:  Tighten the ignition module bolt to 8 N.m (71 lb in).

17. Connect the ignition module electrical connection.