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Diagnostic Procedure

WARNING: This page is about a different car, the 2005 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.
P0300-MULTIPLE CYLINDER MISFIRE DIAGNOSTIC TEST REFERENCE

TEST ACTION APPLICABILITY
1
NOTE: Check for any TSBs that apply to a Misfire condition. Review the vehicle repair history for any misfire condition repairs that have been performed.
Read and record the FREEZE FRAME DATA. Select OBD II MONITORS. Read and record the MIS-FIRE 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 there a misfire present?Yes --> Go To 2No --> Refer to the INTERMITTENT CONDITION SYMPTOM .Perform POWERTRAIN VERIFICATION TEST VER - 5 .		 All
2
NOTE: Anything that affects the speed of the crankshaft can cause a misfire DTC.
NOTE: 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, P/S Pump, Water pump.
  • Misalignment Water pump, P/S 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, Ign coil, etc.
  • Vacuum leaks
  • Restricted Air Induction system or Exhaust system.
  • Internal engine component failures.
Were any of the above conditions present?Yes --> Repair as necessary.Perform POWERTRAIN VERIFICATION TEST VER - 5 .No --> Go To 3		 All
3 Turn the ignition off.
Disconnect the Ignition Coil harness connector and Fuel Injector harness connector of the cylinder being tested.
Ignition on, engine not running.
With the DRBIII®, actuate the ASD Relay.
Using a 12-volt test light connected to ground, probe the (A142) ASD Relay Output circuit at the Ignition Coil harness connector and the (F142) ASD Relay Output circuit at the Fuel Injector harness connector.
Does the test light illuminate brightly?
Yes --> Go To 4
No --> Repair the excessive resistance or short to ground in the (A142), (F142) ASD Relay Output circuit.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .		 All
4 Turn the ignition off.
Remove the Ignition Coil.
Connect the Ignition Coil harness connector.
NOTE: Before continuing, inspect the Ignition Coil for the following conditions. If a problem is found, replace the Ignition Coil. Damage or Carbon Tracking on the Coil or the spark plug insulator boot.

Install a spark tester on the Ignition Coil.
While cranking the engine observe the spark coming from the spark tester.
NOTE: A crisp blue spark that is able to jump the gap of the spark tester should be generated.

Is good spark present?
Yes --> Go To 5
No --> Go To 14
NOTE: Connect the Fuel Injector harness connector before continuing.
 All
5 Turn the ignition off.
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 condition present?
Yes --> Replace the Spark Plug.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 6		 All
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.
Install a fuel pressure gauge.Start the engine and observe the fuel pressure reading.
NOTE: Fuel pressure specification is 407 KPa +/- 34 KPa (59 psi +/- 5 psi).
Choose a conclusion that best matches your fuel pressure reading.Within SpecificationGo To 7Below SpecificationGo To 12Above SpecificationReplace the fuel filter/pressure regulator.Perform POWERTRAIN VERIFICATION TEST VER - 5 .		 All
7
NOTE: Before continuing visually and physically inspect the fuel delivery system for external leaks or damage. Repair/replace as necessary.
Turn the ignition off.
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.
Install special tool #6539 (5/16") or #6631 (3/8") fuel line adapter.Install the fuel pressure gauge.Start the engine and allow the fuel system to reach maximum pressure.Turn the ignition off.
NOTE: Fuel specification is 407 KPa +/- 34 KPa (59 psi +/- 5 psi).
Using special tool #C4390, Hose Clamp Pliers, pinch the rubber fuel line between the fuel pressure gauge and the engine.Monitor the fuel pressure gauge for a minimum of 5 minutes.
NOTE: The pressure should not fall below 241 KPa (35 psi)
Does the Upstream gauge fall below the above specification?Yes --> Replace the leaking Fuel Injector(s).Perform POWERTRAIN VERIFICATION TEST VER - 5 .No --> Go To 8		 All
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.
Remove special tool #C4390.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?Yes --> Go To 9No --> Go To 10
NOTE: Turn the ignition off, remove the Fuel Pressure gauge, and connect the fuel lines before continuing.
 All
9 Check for any of the following conditions/mechanical problems.
ENGINE VACUUM - must be at least 13 inches 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?
Yes --> Repair as necessary.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 16		 All
10 Turn the ignition off.
Disconnect the Fuel Injector harness connector.
Ignition on, engine not running.
NOTE: When a Misfire is detected for a particular cylinder, the PCM will shut down that cylinders Injector Control circuit.

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?
Yes --> Replace the Fuel Injector.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 11		 All
11 Turn the ignition off.
Disconnect the C2 PCM harness connector.
CAUTION: DO NOT PROBE THE PCM HARNESS CONNECTORS. PROBING THE PCM HARNESS CONNECTORS WILL DAMAGE THE PCM TERMINALS RESULTING IN POOR TERMINAL TO PIN CONNECTION. INSTALL MILLER SPECIAL TOOL #8815 TO PERFORM DIAGNOSIS.

Check the Injector Control circuit for an open, short to ground, and short to voltage.
Was a problem found with the Injector Control circuit?
Yes --> Repair the excessive resistance or short to ground in the Injector Control circuit.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 16		 All
12 Turn the ignition off.
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.

Raise vehicle on hoist, and disconnect the fuel pressure line at the fuel pump module.
Install special tool #6539 (5/16") #6631(3/8") fuel line adapter and the fuel pressure gauge between the fuel supply line and the fuel pump module.
Ignition on, engine not running.
With the DRBIII®, actuate the ASD Fuel System test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 407 KPa +/- 34 KPa (59 psi +/- 5 psi).

Is the fuel pressure within specification?
Yes --> Repair or replace fuel supply line as necessary.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 13		 All
13 Turn the ignition off.
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.

Remove the Fuel Pump Module and inspect the Fuel Inlet Strainer.
Is the Fuel Inlet Strainer plugged?
Yes --> Replace the Fuel Pump Inlet Strainer.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No -->
NOTE: Before continuing, check the Fuel Pump Module harness connector terminals for corrosion, damage, or terminal push out. Ensure the ground circuit is operating properly. Repair as necessary. Replace the Fuel Pump Module.

Perform POWERTRAIN VERIFICATION TEST VER - 5 .		 All
14 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 second while observing the test light.
NOTE: The resistance of the primary Ignition Coil on a 3.7L is 0.6 to 0.9 of an ohm at 77°F (25°C).

Does the test light brightly blink/flicker?
Yes --> Replace the Ignition Coil.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 15		 All
15 Turn the ignition off.
Disconnect the C2 PCM harness connector.
CAUTION: DO NOT PROBE THE PCM HARNESS CONNECTORS. PROBING THE PCM HARNESS CONNECTORS WILL DAMAGE THE PCM TERMINALS RESULTING IN POOR TERMINAL TO PIN CONNECTION. INSTALL MILLER SPECIAL TOOL #8815 TO PERFORM DIAGNOSIS.

Check the Coil Control circuit for an open, short to ground, and short to voltage.
Was a problem found with the Coil Control circuit?
Yes --> Repair the Coil Control circuit.
Perform POWERTRAIN VERIFICATION TEST VER - 5 .
No --> Go To 16		 All
16
NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.
Using the schematics 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, view repair.RepairReplace and program the Powertrain Control Module per Service Information.Perform POWERTRAIN VERIFICATION TEST VER - 5 .		 All
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