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HOMESERVICE MANUALSBUICK1996ROADMASTER LIMITEDREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEMS - TESTS W/CODESDIAGNOSTIC TROUBLE CODESDTC P0300-P0308 - ENGINE MISFIRE DETECTEDDIAGNOSTIC PROCEDURES
1996 Buick Roadmaster Limited

Diagnostic Procedures

1996 Buick Roadmaster LimitedSECTION Diagnostic Procedures
  1. Perform On-Board Diagnostic (OBD) System Check. After performing OBD system check, go to next step.
    NOTE: If other DTC(s), except P0131, P0132, P0133, P0151, P0152, P0153, P0171, P0172, P0174, P0175, P1133, P1135, P1136, P1153, P1155 or P1156 are present, diagnose affected DTC(s) first.
  2. Start and operate engine at idle. Using scan tool, select MISFIRE DATA LIST function and monitor MISFIRE CUR. COUNTERS (one counter per cylinder). If any of misfire counter is incrementing, go to next step. If misfire counter is not incrementing, go to step 4).
  3. If only one cylinder is misfiring, go to step 4). If more than one of the cylinders are misfiring, check vacuum hoses for splits, kinks and proper routing. Check for leaks at throttle body, intake manifold, EGR valve or PCV system. Check PCM grounds for proper connection. Check that all injector harness connectors are connected to proper injector/cylinder. Repair as necessary. After repairs, go to step 19). If components are okay, or repair was not necessary, go to step 5).
  4. Turn ignition on, with engine off. Using scan tool, review FREEZE FRAME and/or FAILURE RECORDS data and note parameters. Turn ignition off for about 15 seconds. Start engine and operate vehicle within the conditions required for this diagnostic to run, and as close to conditions recorded in FREEZE FRAME/FAILURE RECORDS as possible. Select DTC function then enter DTC P0300. If scan tool indicates that this test failed this ignition, go to next step. If scan tool does not indicate that this test failed this ignition, see DIAGNOSTIC AIDS.
  5. Turn ignition off. Disconnect ignition coil wire from distributor. Install Spark Tester (J-26792) to coil wire. Crank engine. If spark is present, go to next step. If spark is not present, go to step 12).
  6. Reconnect coil wire to distributor. Disconnect spark plug wire to affected cylinder. Connect spark tester to spark plug wire. Crank engine. If spark is present, go to next step. If spark is not present, go to step 7).
  7. Remove and check spark plug from affected cylinder. If spark plug is okay, go to next step. If spark plug is fouled, go to step 9).
  8. Swap spark plug to another cylinder that is operating properly. Operate vehicle under dame conditions that the misfire occurred. If misfire moved with spark plug, go to step 17). If misfire is still present at original cylinder, perform fuel injector coil test. See SYSTEM/COMPONENT TESTS article.
  9. If spark plug is fouled with oil or coolant, diagnose mechanical engine problem. If spark plug is not fouled, go to next step.
  10. Check spark plug for signs of cracks, wear or improper gap. If spark plug is faulty, go to next step. If spark plug is okay, go to step 18).
  11. Replace or re-gap spark plug. After replacing or re-gapping spark plug, go to step 19).
  12. Using DVOM, check coil wire resistance. Resistance should be about 10,000 ohms. If resistance is as specified, go to next step. If resistance is not as specified, go to step 14).
  13. Replace ignition coil. After replacing ignition coil, go to step 19).
  14. Replace coil wire. After replacing coil wire, go to step 19).
  15. Using DVOM, check spark plug wire to affected cylinder(s). Resistance should be about 10,000 ohms. If resistance is as specified, go to step 17). If resistance is not as specified, go to next step.
  16. Replace faulty spark plug wire(s). After replacing spark plug wire(s), go to step 19).
  17. Replace faulty spark plug(s). After replacing spark plug(s), go to step 19).
  18. Check distributor for carbon tracking, contamination, moisture or physical damage. Replace as necessary. If problem is corrected, go to next step. If problem is still present, check engine for mechanical problems.
  19. If complaint was due to MIL flashing, perform DTC P0420 or P0430 test. If MIL was not flashing, go to next step.
  20. Using scan tool, select DTC, CLEAR INFO function. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0300. Operate vehicle within conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool did not indicate that this test ran and passed, repeat step 2).
  21. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.