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HOMESERVICE MANUALSBUICK1989SKYHAWK S/E, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEM2.0L(K)/2.5L TESTS W/CODESDESCRIPTIONCHART C4D1 - "DIS" MISFIRE AT IDLE (2.5L A, N BODY)
1989 Buick Skyhawk S/E, Standard

Chart C4D1 - "Dis" Misfire At Idle (2.5L A, N Body)

1989 Buick Skyhawk S/E, StandardSECTION Chart C4D1 - "Dis" Misfire At Idle (2.5L A, N Body)

The Direct Ignition System (DIS) uses a waste spark system for spark distribution. The ignition module triggers the ignition coils and fires the spark plugs in pairs (1-4 and 2-3). While one cylinder is at TDC compression, the opposite cylinder is at TDC exhaust. The exhaust cylinder requires less voltage to fire the spark plug, leaving the remainder of the high voltage to fire the compression cylinder spark plug.

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. If misfire condition exists only under load, CHART C4D-2 must be used. RPM should drop equally on all plug leads.
  2. A Spark Plug Tester (ST-125), requiring a minimum of 25,000 volts to fire, must be used because it is essential to verify adequate available secondary voltage at the spark plug.
  3. If the spark jumps the test gap after the opposite plug wire is grounded, it indicates excessive resistance in the plug which was by-passed. Also check for carbon deposits in spark plug boots.
  4. Excessive wire resistance or faulty connections could cause the coil to be damaged. If carbon tracking exists, replace coil.
  5. If no spark conditions follows the suspected coil, replace coil. Otherwise, the ignition module is the cause of no spark. This test could also have been performed by substituting a known good coil for the suspected coil.
Fig 1: Chart C4D1 Schematic, "DIS" Misfire At Idle (2.5L)
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