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Home >> Saturn >> 2003 >> L300 >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures >> Preliminary Inspection & Adjustments >> Mechanical Inspection >> Engine Compression Test
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Engine Compression Test

CAUTION: Cylinder leakage checks must be performed with the vehicle in park or neutral and the parking brake applied.

If there is a lack of power, excessive oil consumption, or poor fuel economy, etc., cylinder leakage can be measured.

  1. Start the engine and allow it to reach normal operating temperature.
  2. Shut the engine OFF and disconnect the electronic ignition module wiring harness plugs.
  3. Remove electronic ignition modules. See ELECTRONIC IGNITION MODULE under IGNITION SYSTEMS in REMOVAL & INSTALLATION - SATURN "L" SERIES article.
  4. Remove all 6 spark plugs.
  5. Insert the Gauge Bar Set into the spark plug hole.
  6. Fully open the throttle.
    7. NOTE: 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. 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. While cranking the engine, measure the compression. The cylinder should puff or the compression gauge needle should bounce a minimum of 10 times.
    8. NOTE: No compression in 2 or more cylinders is an indication of damaged timing chain sprockets, camshafts, and 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 100 psi (690 kPa).
  8. Repeat the test procedure for the remaining cylinders and record all readings.

    1. Normal Compression

      Builds up quickly and evenly to specification in each cylinder.

    2. Low Compression

      Damaged piston, rings, valves, or head gasket.

    3. High Compression

      Carbon on pistons/combustion chamber.
  9. If the cylinder compression in one or more cylinders is low, pour approximately 0.5 oz. (15 ml) of engine oil into the cylinder through the spark plug hole. A cylinder leakage test can be performed to further determine the location of the compression leakage if one or more cylinders are extremely low. See CYLINDER LEAKAGE TEST  .
  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 gauge or spark plugs installed.
  12. Repeat previous steps  4 - 7 and measure the compression.
  13. If adding oil increases the compression readings the piston rings are leaking. Compression is low on the first stroke (first gauge puff) and tends to build up on following strokes, but does not reach normal compression pressure. If compression improves considerably with oil, inspect for worn or broken rings and piston ring land wear.
  14. If adding oil does not increase the compression:

    1. 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.

    2. Head Gasket

      If 2 adjacent cylinders have lower than normal compression and adding oil to the cylinder does not improve compression.
  15. Remove the compression tester and install the spark plugs. Tighten spark plugs to 18 ft. lbs. (25 N.m).
  16. Install the electronic ignition modules. Tighten electronic ignition module bolts to 71 INCH lbs. (8 N.m).
  17. Connect the electronic ignition module electrical connections.
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