Compression Check

TOOLS REQUIRED 

• SA9127E Gauge Bar Set

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

1. Start the engine and allow it to reach normal operating temperature.
2. Shut engine Off and disconnect the ignition module wiring harness plugs.
3. Remove ignition modules, and remove all six spark plugs. Refer to REMOVAL & INSTALLATION (3.0L) .
   Fig 1: Removing Ignition Modules

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4. Insert a compression gauge bar kit Gauge Bar Set SA9127E into the spark plug hole.
   Fig 2: Installing Compression Gauge Into Cylinder

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5. Fully open throttle.

   IMPORTANT:  Always use a fully charged battery to obtain engine revolutions of 250 RPM minimum. Do not crank the engine for more than 15 seconds.

6. While cranking engine, measure compression. The cylinder should puff or compression gauge 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 gauge.

7. Repeat the previous steps for each cylinder.

   IMPORTANT:  No compression in two or more cylinders is an indication of damaged timing chain sprockets, camshaft(s), crankshaft key or pins.

   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, ring(s), valve(s), or head gasket.
   • High Compression: Carbon on pistons/combustion chamber.

   IMPORTANT:  A cylinder leakage test can be performed to further determine location of compression leakage if one or more cylinders are extremely low.

8. If the cylinder compression in one or more cylinders is low, pour a small amount (one teaspoon) of engine oil into the cylinder through the spark plug hole.
   Fig 3: Adding Oil To Cylinder To Perform Compression Test

   Courtesy of GENERAL MOTORS CORP.
9. Place a shop towel over plug holes to capture any oil mist/spray.
   Fig 4: Placing Shop Towel Over Spark Plug Holes

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10. Crank the engine over for a few seconds without the compression gauge or spark plugs installed.
11. Repeat previous steps  4- 7 and measure compression.
12. If adding oil increases compression readings:
    • Pistons Rings Leaking: Compression low on first stroke (first gauge puff) and tends to build up on following strokes, but does not reach normal compression pressure. Improves considerably with oil. Inspect for worn or broken ring(s) and piston ring land wear (pound out).
    Fig 5: Looking For Worn Or Broken Piston Rings

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13. If adding oil does not increase compression:
    • Valves or Head Gasket Leaking: Compression low on first stroke. Does not tend to build up on following strokes. Does not improve with adding oil to cylinder.
      Fig 6: Checking For Leaking Valve Or Head Gasket

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    • Head Gasket: Two adjacent cylinders have lower than normal compression and adding oil to the cylinder does not improve compression.
      Fig 7: Checking For Break In Head Gasket

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14. Remove compression tester and install spark plugs.

    Torque: 

    Spark Plug: 25 N.m (18 ft-lbs) 

15. Install ignition module.

    Torque: 

    Ignition Module Bolt: 8 N.m (71 in-lbs) 

    Fig 8: Reinstalling Ignition Module

    Courtesy of GENERAL MOTORS CORP.
16. Connect ignition module electrical connection.