SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 79 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.4L)DIAGNOSTIC TESTSDTC P0300-P0306: ENGINE MISFIRE DETECTEDDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, 2002 Chevrolet Monte Carlo, and 2002 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
  2. Are any other DTCs set? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Start and idle the engine. Review and record scan tool Freeze Frame data. Operate the vehicle to duplicate the conditions present when the DTC was set, as defined by the Freeze Frame data. Monitor the scan tool Misfire Current Cyl # display for each cylinder. Is Misfire Current # display incrementing for any cylinder, indicating a misfire currently occurring? If yes, go to step  5. If no, go to next step.
  4. View the Misfire History Cyl # display on the scan tool. Does Misfire History Cyl # display a very large value for more than one cylinder? If yes, go to next step. If no, go to step  8.
  5. Are the misfire values displayed on the Misfire History Cyl # related to companion cylinders (i.e., 1/4, 2/5, 3/6)? If yes, go to step  9. If no, go to next step.
  6. Inspect the following items: Excessive vibration from sources other than the engine. The following are possible sources of vibration: The drive belts, variations in brake rotor thickness, vacuum hoses, crankcase ventilation system for vacuum leaks, throttle body and intake manifold for vacuum leaks, PCM grounds and spark plug wires. Measure the resistance of the spark plug wires associated with the misfiring cylinders. Check EGR valve to exhaust manifold gasket for signs of leaks or damage. Check fuel for contamination. Did you find and correct the condition? If yes, go to step  18. If no, go to next step.
  7. Install a fuel pressure gauge. Turn ignition on, with engine off. With a scan tool, command the fuel pump ON. Monitor the fuel pressure with the fuel pump running. Does the fuel pressure gauge indicate that the fuel pressure is within 52-59 psi (358-405 kPa). If yes, go to next step. If no, diagnose fuel system.
  8. Test for proper fuel injector operation. Refer to FUEL INJECTOR BALANCE TEST under FUEL SYSTEMS in appropriate SYSTEM & COMPONENT TESTING article. Did you find and correct the condition? go to step  18. If no, go to next step.
    9. NOTE: If carbon tracking is apparent at either end of the spark plug wires, replace the affected ignition wire and the associated ignition coil.
  9. Inspect the ignition wires and coils associated with the misfiring cylinders for the following conditions: Carbon tracking/arching and damage, spark plug wires connected to incorrect cylinders at coil and spark plug, or poor terminal connections at coil and spark plug. If a problem is found, repair as necessary. Did you find and correct the condition? If yes, go to step  18. If no, go to next step.
  10. Install the J-26792 Spark Tester at the spark plug end of the ignition wire associated with the misfiring cylinders. Jumper the spark plug end of the companion cylinder ignition wire to engine ground. The companion cylinder is the cylinder that shares the same ignition coil (i.e., 1/4, 2/5, 3/6). Crank the engine while observing the spark tester. Is spark present? If yes, go to step  15. If no, go to next step.
  11. Measure the resistance of the ignition wires associated with the cylinders that were indicated as misfiring. Replace the ignition wires if the resistance measures more than 3000 ohms per foot. Did you find and correct the condition? If yes, go to step  18. If no, go to next step.
  12. Measure the resistance of the secondary ignition coils associated with the misfiring cylinders. If the resistance is not 5-8 k/ohms, replace the malfunctioning ignition coils. Did you find and correct the condition? If yes, go to step  18. If no, go to next step.
  13. Remove the ignition coils associated with the misfiring cylinders. Inspect the coils for signs of carbon tracking, cracks or other damage. Did you find and correct the condition? If yes, go to step  18. If no, go to next step.
  14. Leave the ignition coils disconnected. Connect a test light across the ignition module primary circuit terminals. Remove the fuel injector fuse. Observe the test light while cranking the engine. Does the test light blink? If yes, go to step  16. If no, go to step  17.
  15. Remove the spark plugs from the cylinders associated with the misfire. Inspect the spark plugs. Did you find and correct the condition? If yes, go to step  18. If no, see DIAGNOSTIC AIDS .
  16. Replace the ignition coils associated with the misfiring cylinders. After repairs, go to step  18.
  17. Replace the ignition control module. After repairs, go to next step.
  18. Review and record Freeze Frame data. Clear DTCs. Start and idle the engine. Operate vehicle to duplicate the conditions present when the DTC was set, as defined by the Freeze Frame data. Monitor the scan tool Misfire Current Cyl # display for each cylinder. Is Misfire Current Cyl # display incrementing for any cylinder, indicating a misfire currently occurring? If yes, diagnose for an engine mechanical problem. If no, system is okay.
RENDER: 1.0x

NO RELATED

Recommended Tools & Savings

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Affiliate disclosure: We may earn a commission at no extra cost to you.

Shop Manual-Ready ToolsFree Quote CheckQuote Shield Pro

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.