P0300 Multiple Cylinder Misfire

P0300 Multiple Cylinder Misfire - With the engine running, the adaptive numerator has been successfully updated. Set condition: The variation in crankshaft speed between each cylinder exceeds a calibrated value, based on the engine rpm load.

Default Action:

The MIL light will illuminate.

Possible Causes:

• ASD Relay output circuit.
• Injector control circuit.
• Coil control circuit.
• Ignition wire.
• Spark Plug.
• Ignition Coil.
• Fuel delivery system issue.
• Fuel Injector.
• Engine Mechanical.
• Powertrain control Module (PCM).

Diagnose code P0300 Multiple Cylinder Misfire

Step 1. Check for an active DTC

Diagnose and repair any other active component or circuit DTC before continuing with this test.

Turn the ignition on, with the scan tool, read DTC. Operate the vehicle under conditions similiar to which the DTC was set. With the scan tool, view the misfire monitor to see which cylinders are effected.

Are there excessive misfires shown or is the DTC pending at this time?

Yes, go to step 2.

No, perform the the PCM intermittent condition.

Step 2. Visual Inspection

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 of the water pump, P/S pump, A/C compressor pulleys.
• Corroded PCM power and ground circuit.
• Improper CKP, CMP, and TP sensor mounting.
• Poor connector/terminal to component connection for CKP sensor, CMP sensor, MAP sensor, TP sensor, fuel injector, ignition coil, etc.
• Vaccum leaks.
• Restricted exhaust system.
• Restricted Air induction system.
• Internal engine component failures.
• Overfilled engine oil or transmission.
• Extremely out of balance tires or wrapped brake rotors.

Were any of the above conditions present?

Yes, repair as necessary.

No, go to step 3.

Step 3. Ignition System operation

Turn the ignition off.

Remove the ignition coils from the cylinders that were counting misfires.
Connect the ignition coil harness connectors.
Disconnect the fuel injector harness connector of the cylinder being tested. Install a sprak tester on the ignition coil. 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 at the coil on plug and the end of the ignition wire.

Is good spark present on all cylinders?

Yes, go to step 4.

No, go to step 13.

Step 4. Spark Plug

Turn the ignition off, remove spark plug, inspect the spark plug for the following conditions:

• Cracks.
• Carbon tracking.
• Foreign material.
• Gap size out of specifications.
• Loose or broken 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?

Yes, replace the spark plug.

No, go to step 5.

Step 5. Fuel Pressure check

Install the Decay Tool to the fuel rail. Start the engine and observe the fuel pressure reading. Fuel pressure specification is 400 kPa +/- 14 kPa (58 psi +/- 2 psi).

Choose a conclusion that best matches your fuel pressure reading.

Within specification, go to step 6.

Below specification, go to step 11.

Above specification, replace the fuel pressure regulator.

Step 6. Fuel pressure leak down
Perform the Fuel pressure leak down test.

Were any repairs made after performing the leak down test?

Yes, testing complete.
No, go to step 7.

Step 7. Fuel injector operation
Open the fuel system isolation valve. Start the engine and allow the fuel pressure to reach maximum pressure. Turn the ignition off, turn the ignition on. With the scan tool, 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 step 8.
No, go to step 9.

Note: Turn the ignition off, remove the fuel pressure delay tester and connect the fuel lines before continuing.

Step 8. Engine Mechanical

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.
• Cylinder leakage test - 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.
• Valve springs - cannot be weak or broken.
• Cam lobes - must not be worn excessively.
• Cam shaft - must not be worn or have excessive and play.

Are there any engine mechanical problems?

Yes, repair as necessary.

No, go to step 17.

Step 9. Fuel injector control

Turn the ignition off, disconnect the fuel injector harness connector of the misfiring cylinder. Turn the ignition on.

Install an injector noid night. With the scan tool, actuate the fuel injector.

Does the noid light blink/flicker during the actuation?

Yes, verify that there is good pin to terminal contact in the related injector and Powertrain Control Module (PCM) connectors. Replace the fuel injector if problems are not found in the connectors.

No, go to step 10.

Step 10. Injector control circuit

Turn the ignition off, remove the injector noid night. 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?

Yes, repair the excessive resistance or short in the injector control circuit.

No, go to step 17.

Step 11. Fuel supply line restricted

Turn the ignition off, raise the vehicle on a hoist and disconnect the fuel pressure line at the fuel pump module. Install the decay tool, turn the ignition on, with the scan tool, actuate the ASD fuel system test and observe the fuel pressure gauge. Fuel pressure specification is 400 kPa +/- 14 kPa (58 psi +/- 2 psi).

Is the fuel pressure within specification?

Yes, repair or replace fuel supply line as necessary.

No, go to step 12.

Step 12. Fuel pump inlet strainer

Turn the ignition off, remove the fuel pump module and inspect the fuel inlet strainer.

Is the fuel inlet strainer plugged?

Yes, replace the fuel pump inlet strainer.

No, before continuing, check the fuel pump module harness connector terminals for corrosion, damage or terminal push out.

Step 13. ASD relay output circuit

Turn the ignition off, disconnect the coil on plug harness connector. Turn the ignition on, with the scan tool, actuate the ASD relay. Using a 12-volt test light conencted to ground, probe the ASD output circuit in the coil on plug harness connector.

Does the test light illuminate brightly?

Yes, go to step 14.

No, repair the open or short to ground in the ASD relay output circuit between the TIPM/BCM and coil harness connector.

Step 14. Ignition coil

Using a 12-volt test light connected to a 12 volt source, probe the coil control circuit. Crank the engine for 5.0 seconds while observing the test light.

What is the condition of the test light while cranking the engine?

Brightly blinking, replace the ignition coil.

On constantly, go to step 15.

Off constantly, go to step 16.

Step 15. Coil control circuit shorted to ground

Turn the ignition off, disconnect the PCM harness connector. Measure the resistance between ground and the coil control circuit in the coil on plug harness connector.

Is the resistance below 10k Ohms?

Yes, repair the short to ground in the coil control circuit.

No, go to step 17.

Step 16. Coil control circuit open

Turn the ignition off, disconnect the PCM harness connector. Measure the resistance of the coil control circuit from the coil on plug harness connector to the PCM harness connector.

Is the resistance below 3.0 Ohms?

Yes, go to step 17.

No, repair the open in the coil control circuit.

Step 17. Powertrain Control Module (PCM)

Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the ignition coil and the Powertrain Control Module (PCM).

Look for any chafed, pierced, pinched, or partially broken wires. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact line the ignition coil and Powertrain Control Module (PCM) connectors. Refer to any Service Bulletins that may apply.

Were there any problems found?

Yes, repair as necessary.

No, replace and program the Powertrain Control Module (PCM). Perform the Powertrain Verification Test.

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