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Home >> Jeep >> 2003 >> Wrangler SE, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0300: Multiple Cylinder Misfire, DTC P0301: Cylinder #1 Misfire, DTC P0302: Cylinder #2 Misfire, DTC P0303: Cylinder #3 Misfire, DTC P0304: Cylinder #4 Misfire, DTC P0305: Cylinder #5 Misfire (4.0L) Or DTC P0306: Cylinder #6 Misfire (4.0L) >> Testing

DTC P0300: Multiple Cylinder Misfire, DTC P0301: Cylinder #1 Misfire, DTC P0302: Cylinder #2 Misfire, DTC P0303: Cylinder #3 Misfire, DTC P0304: Cylinder #4 Misfire, DTC P0305: Cylinder #5 Misfire (4.0L) Or DTC P0306: Cylinder #6 Misfire (4.0L): Testing

    NOTE: Check for any TSB's that apply to a Misfire condition. Review the vehicle repair history for any misfire condition repairs that have been performed.
  1. Turn ignition on, engine not running. With the DRBIII®, select DTCs and RELATED FUNCTIONS. Read and record the FREEZE FRAME DATA. Select OBD-II MONITORS. Read and record the MISFIRE SIMILAR CONDITIONS WINDOW DATA. With these screens, attempt to duplicate the condition(s) that has set this DTC. When the vehicle is operating in the SIMILAR CONDITIONS WINDOW, refer to the WHICH CYLINDER IS MISFIRING screen. Observe the WHICH CYLINDER IS MISFIRING screen for at least one minute. Is the DRBIII® counting misfires at this time? If yes, go to next step. If no, go to INTERMITTENT CONDITION  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: Anything that affects the speed of the crankshaft can cause a misfire DTC. When a Misfire is detected for a particular cylinder, the PCM will shut down that cylinders Injector Control circuit.
    • Visually inspect the engine for any of the following conditions.
    • Worn serpentine belt
    • Binding Engine-Driven accessories: A/C Compressor, Power Steering Pump, Water pump.
    • Misalignment Water pump, Power Steering Pump and A/C Compressor pulleys
    • Corroded PCM power and ground circuits.
    • Improper CKP, CMP, MAP, and TP Sensor mounting
    • Poor connector/terminal to component connection. i.e., CKP sensor, Fuel Injector, Ignition coil, etc.
    • Vacuum leaks
    • Restricted Air Induction system or Exhaust system.
    Were any of the above conditions present? If yes, repair as necessary. If no, go to the next step.
  3. Turn the ignition off. Disconnect the Ignition Coil harness connector. Disconnect the Fuel Injector harness connector. Ignition on, engine not running. With the DRBIII®, actuate the ASD Relay. Using a 12-volt test light connected to ground, probe the (F42) ASD Relay Output circuit at the Ignition Coil harness connector and Fuel Injector harness connector. Does the test light illuminate brightly? If yes, go to the next step. If no, repair the excessive resistance or short to ground in the (F42) ASD Relay Output circuit.
  4. Turn the ignition off. Disconnect the Ignition wire from the spark plug. Disconnect the Fuel Injector harness connector of the cylinder being tested. Before continuing inspect the ignition wire for damage or carbon tracking. Replace as necessary. Install a spark tester on the ignition wire. While cranking the engine observe the spark coming from the spark tester. A crisp blue spark that is able to jump the gap of the spark tester should be generated. Is good spark present? If yes, go to the next step. If no, go to step  12.
  5. Turn the ignition off and remove the Spark Plug. Inspect the Spark Plug for the following conditions:
    • Cracks
    • Carbon Tracking
    • Foreign Material
    • Gap size out of specifications
    • Loose or broke electrode
      NOTE: Lightly tap the bottom of the spark plug on a solid surface. The electrode in the spark plug should not move.
    Were any of the above conditions present? If yes, replace the spark plug. If no, go to the next step.
    6. WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released.
  6. Install a fuel pressure gauge. Start the engine and observe the fuel pressure reading. Fuel pressure specification is 44-54 psi (300-368 kPa). If your fuel pressure reading is within specification, go to the next step. If it is below specification, the test is complete. If it is above specification, replace the fuel filter/pressure regulator.
    7. WARNING: The fuel system is under a constant pressure even with the engine off. Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released.
    NOTE: Before continuing visually and physically inspect the fuel delivery system for external leaks or damage. Repair/replace as necessary.
  7. Turn the ignition off. Install 5/16" Fuel Line Adapter (6539). Install the fuel pressure gauge. Start the engine and allow the fuel system to reach maximum pressure. Turn the ignition off. Fuel specification is 44-54 psi (300-368 kPa). Using Hose Clamp Pliers (C4390), pinch the rubber fuel line between the fuel pressure gauge and the engine. Monitor the fuel pressure gauge for a minimum of 5 minutes. The pressure should not fall below 35 psi (241 kPa). Does the Upstream gauge fall below the above specification? If yes, replace the leaking Fuel Injector(s). If no, go to the next step.
    8. WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released.
    CAUTION: After each Fuel Injector actuation, start the engine to clean the cylinder of fuel. Failure to do so could cause engine damage.
    NOTE: Turn the ignition off, remove the fuel pressure gauge, and connect the fuel lines before continuing.
  8. Remove hose clamp pliers. Start the engine and allow the fuel pressure to reach maximum pressure. Ignition on, engine not running. Using the DRBIII®, actuate the Fuel Injector for the cylinder that indicated the misfire. Monitor the fuel pressure gauge. Does the fuel pressure gauge indicate a drop in fuel pressure? If yes, go to the next step. If no, go to step  10.
  9. Check for any of the following conditions/mechanical problems.
    • Engine vacuum must be at least 13 in. Hg in neutral.
    • Engine valve timing must be within specifications.
    • Engine compression must be within specifications.
    • Engine exhaust system must be free of any restrictions or leaks.
    • Engine PCV system must flow freely.
    • Torque converter stall speed must be within specifications.
    • Power brake booster no internal vacuum leaks.
    • Fuel must be free of contamination.
    • Cam lobes must not be worn excessively.
    • Cylinder leakage test must be within specifications.
    • Valve springs cannot be weak or broken.
    Are there any engine mechanical problems? If yes, repair as necessary. If no, go to step  15.
    10. NOTE: When a Misfire is detected for a particular cylinder, the PCM will shut down that cylinder's Injector Control circuit.
  10. Turn the ignition off. Disconnect the Fuel Injector harness connector. Turn the ignition on, engine not running. With the DRBIII®, erase DTCs. Using a 12-volt test light connected to 12-volts, probe the Injector Control circuit. With the DRBIII®, actuate the Fuel Injector. Does the test light blink/flicker? If yes, replace the Fuel Injector. If no, go to the next step.
  11. Turn the ignition off. Disconnect the PCM harness connectors. Check the Injector Control circuit for an open, short to ground, and short to voltage. Was a problem found with the Injector Control circuit? If yes, repair the excessive resistance or short to ground in the Injector Control circuit. If no, go to step  15.
  12. Turn the ignition off. Remove the ignition wire. Measure the resistance of the ignition wire. Is the resistance below 10 k/ohms? If yes, go to the next step. If no, replace the Ignition Wire.
  13. Turn the ignition off. Disconnect the Ignition Coil harness connector. Remove the Fuel Pump Relay or ASD Relay. Using a 12-volt test light connected to 12-volts, probe the Ignition Coil Control circuit. Crank the engine for 5 seconds while observing the test light. The resistance of the primary Ignition Coil on a 2.4L is 0.51 to 0.61 ohm and the 4.0L Primary Coil Rail resistance is 0.53 to 0.63 ohm at 77°F (25°C). Does the test light brightly blink/flicker? If yes, replace the Ignition Coil. If no, go to the next step.
  14. Turn the ignition off. Disconnect the PCM harness connectors. Check the Coil Control circuit for an open, short to ground, and short to voltage. Was a problem found with the Coil Control circuit? If yes, repair the Coil Control circuit. If no, go to the next step.
    15. NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.
  15. Using the wiring diagram as a guide, inspect the wire harness and connectors. Pay particular attention to all power and ground circuits. If there are no possible causes remaining, replace the Powertrain Control Module (PCM). Program the new PCM. See PROGRAMMING .
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