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For 1990-2009 cars only
Makes 🢒 Buick 🢒 1999 🢒 Riviera 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 DTC P0300 Engine Misfire Detected

Circuit Description

The PCM has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the Ignition Control Module. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring.

If 2% or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 22 or more misfires are detected during any 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible three-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the MIL will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Running the DTC
    •  No VSS, Transmission, TP sensor, Fuel trim, Fuel injector, MAP sensor, EVAP, HO2S, IC/Bypass line monitor, EGR flow, EGR pintle position, ECT sensor, CKP sensor CMP sensor, or MAF sensor DTCs set.
    •  Engine speed between 550 and 5800 RPM.
    •  System voltage between 9 and 18 volts.
    •  The ECT indicates an engine temperature between -6°C (21°F) and 120°C (248°F).
    •  Throttle angle steady.

Action Taken When the DTC Sets
    • The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure.
    • The PCM writes the conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC
    • The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using a scan tool.

Diagnostic Aids

The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as Test failed since code clear, the misfire history counters (Misfire Hist #1 - #6) will still contain a value that represents the level of misfire for each cylinder.

A misfire DTC may set if components that affect the Crankshaft Position Sensor have recently been replaced, and the CKP System Variation Learn Procedure has not been performed.

If the diagnostic table does not identify a problem then perform the Crankshaft Position System Variation Learn .

The Crankshaft Position Variation Learn Procedure should be performed if any of the following conditions are true:

    • The PCM has been replaced.
    • DTC P1336 is set.
    • The Engine has been replaced.
    • The Crankshaft has been replaced.
    • The Crankshaft Harmonic Balancer has been replaced.
    • The Crankshaft Position Sensor has been replaced.

The scan tool displayed misfire counter values (Misfire Hist. #1 through #6) can be useful in determining whether the misfire is isolated to a single cylinder or to a cylinder pair (cylinders that share an ignition coil-1/4, 2/5, 3/6.) If the largest amount of activity is isolated to a cylinder pair, check for the following conditions:

If the misfire is random, check for the following conditions:

    • Secondary Ignition Wires.
        Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance.
        If any spark plug wire resistance is greater than the specified value, replace the affected spark plug wire(s).

VIN 1 Spark Plug Wires Resistance
3280 - 4921 ohms per meter (1000 - 1500 ohms per /foot 1K - 1.5K ohms per foot)

    • Damaged Or Malfunctioning Ignition Coil.
        Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be between the specified values.

Ignition Coils Secondary Resistance
5000 - 8000 ohms (5K - 8K ohms).

    • Substitute a Known Good Coil.
        Switch ignition coils and retest. If the misfire follows the coil, replace the ignition coil. Refer to Ignition Coil Replacement .
    • System Grounds.
        Ensure all connections are clean and properly tightened. Refer to Intermittents and Poor Connections Diagnosis .
    • Mass Air Flow sensor.
        A Mass Air Flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the fail records conditions. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement .
    • Loss of EBCM/EBTCM Serial Data.
        If the PCM stops receiving data from the EBCM/EBTCM, DTC P0300 can set due to a loss of rough road data. Check for stored ABS/TCS DTCs, especially DTCs related to a serial data malfunction. Refer to Diagnostic System Check - ABS .
    • Air Induction System.
        Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.
    • Fuel Pressure.
        Perform a fuel system pressure test. A malfunctioning fuel pump, plugged filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Pressure Test (VIN 1) .
    • Fuel injector(s).
        Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .
    • Contaminated Fuel.
        Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) and Alcohol/Contaminants-in-Fuel Diagnosis .
    • EGR System.
        Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
    • Extended Idle.
        Excessive open loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to be set, review the freeze frame and fail records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures less than 80°C (176°F), the condition described above is very likely the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 gm/s.

Important:: If the level of misfire was sufficient to cause possible catalyst damage (if the MIL was flashing), ensure that the DTC P0420 test is completed and passed after verifying the misfire repair.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Test Description

The numbers below refer the step numbers on the diagnostic table.

  1. A malfunctioning injector circuit, crankshaft position system variation not learned condition, or incorrect rough road data from the EBCM may cause a misfire DTC to be set. If any of the indicated DTCs are set with DTC P0300, diagnose and repair the other DTC before using the DTC P0300 table.

    2.

  2. The Misfire Current Cyl # display may normally display a small amount of activity (0 - 10 counts) but should not steadily increment during an entire 200 revolution test sample period.

    3.

  3. Depending on the cause of the misfire, the Misfire History Cyl # counter will display a very large number for the misfiring cylinder(s); values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinder(s). When investigating a misfire, always start with items associated with the cylinder(s) that has the largest number of counts stored in the Misfire History Cyl # counter.

    4.

  4. If the misfiring cylinders are companion cylinders, the condition is most likely linked to the ignition system.

    5.

  5. Check for poor terminal connection, grooves, corrosion, pitting, loose fit.

    6.

  6. Check for the following conditions that may contribute to the engine misfire: Engine oil pressure, Damaged accessory drive belt or pulley, Damaged driven accessory (generator, water pump, drive belt tensioner, etc), Loose or broken motor mount(s).

DTC P0300 - Engine Misfire Detected

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Are any other DTCs set?

--

Go to Powertrain On Board Diagnostic (OBD) System Check

Go to Step 3

3

  1. Start and idle the engine.
  2. Review and record scan tool Freeze Frame data.
  3. Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame data).
  4. Monitor the scan tool Misfire Current Cyl # display for each cylinder.

Is Misfire Current # display incrementing for any cylinder (indicating a misfire currently occurring)?

--

Go to Step 5

Go to Step 4

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?

--

Go to Step 5

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)?

--

Go to Step 9

Go to Step 6

6

  1. Perform a visual inspection of the following areas:
    2. •  The vacuum hoses for improper connections and damage. Refer to the Emission Hose Routing Diagram .
    •  Spark plug wire connections at the coils and at the spark plugs.
    •  Engine and PCM grounds. Ensure that the connections are in their proper locations, clean and tight. Refer to Ground Distribution Schematics .
    •  EGR to intake manifold piping for proper connections and signs of damage.
    • Visually and physically inspect the PCV Valve for improper installation and damaged o-rings. Refer to Crankcase Ventilation System Inspection .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 22

Go to Step 7

7

Check the fuel pressure. Refer to Fuel System Pressure Test (VIN 1) .

Is the fuel pressure within the specified values?

333-376 kPa (48-55 psi)

Go to Step 8

Go to the Fuel System Pressure Test (VIN 1)

8
  1. Check for proper fuel injector operation. Refer to the Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 22

Go to Step 9

9
  1. Visually inspect the ignition wire(s) and coil(s) associated with the misfiring cylinder(s) for the following condition(s):
    2. •  Carbon tracking/arching and damage.
    •  Connected to the incorrect cylinders at the coil and spark plug.
    •  Poor terminal connections at the coil and spark plug.
  2. If a problem is found, repair as necessary.

Important: :  If carbon tracking is apparent at either end of the spark plug wires, replace the affected ignition wire and the associated ignition coil. Refer to Spark Plug Wire Harness Replacement and Ignition Coil Replacement .

Was a problem found?

--

Go to Step 22

Go to Step 10

10
  1. Install J 26762 Spark Tester at the spark plug end of the ignition wire associated with the misfiring cylinder(s).
  2. 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).
  3. Crank the engine while observing the spark tester.

Is spark present?

--

Go to Step 15

Go to Step 11

11
  1. Measure the resistance of the ignition wire(s) associated with the cylinder(s) that were indicated as misfiring.
  2. Replace the ignition wire(s) if the resistance measures greater than the specified value. Refer to Spark Plug Wire Harness Replacement .

Was a problem found?

1K-5K ohms (1000 -1500 ohms per foot)

Go to Step 22

Go to Step 12

12
  1. Measure the resistance of the secondary ignition coil(s) associated with the misfiring cylinder(s).
  2. If the resistance is not between the specified values, replace the malfunctioning ignition coil(s). Refer to Ignition Coil Replacement

Was a problem found?

5K-8K ohms (5,000-8,000 ohms)

Go to Step 22

Go to Step 13

13
  1. Remove the ignition coil(s) associated with the misfiring cylinder(s).
  2. Inspect the coil(s) for signs of carbon tracking, cracks or other damage.
  3. If a problem is found, replace the affected coil(s). Refer to Ignition Coil Replacement .

Was a problem found?

--

Go to Step 22

Go to Step 14

14
  1. Leave the ignition coil(s) disconnected.
  2. Connect a test lamp across the ignition module primary circuit terminals.
  3. Remove the fuel injector fuse.
  4. Observe the test lamp while cranking the engine.

    5. Important: :  Be sure to reinstall the fuel injector fuse.

Does the test lamp blink?

--

Go to Step 20

Go to Step 21

15

  1. Remove the spark plugs from the cylinder(s) associated with the misfire. Refer to Spark Plug Replacement .
  2. Visually inspect the spark plug electrodes for excessive fouling.

Was a problem found?

--

Go to Oil Consumption Diagnosis

Go to Step 16

16

  1. Visually inspect the spark plugs for the following condition(s):
    2. •  Carbon tracked, cracked or other damage to the insulator.
    •  Electrode damage or incorrect gap.

        Important: :  If carbon tracking is apparent on any of the spark plugs, replace the affected spark plug(s) and the associated ignition wire(s).

  2. If a problem is found, replace the affected spark plug(s). Refer to Spark Plug Replacement .

Was a problem found?

--

Go to Step 22

Go to Step 17

17

Reinstall the spark plug.

  1. Check for engine mechanical problems.
  2. Base engine mechanical problems. Refer to Engine Compression Test .
  3. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 22

Go to Step 18

18
  1. If condition occurs while driving, check for transaxle TCC problems.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 22

Go to Step 19

19

  1. Check for contaminants in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 22

Go to Diagnostic Aids

20

Replace the ignition coil(s) associated with the misfiring cylinder(s). Refer to Ignition Coil Replacement .

Is the action complete?

--

Go to Step 22

--

21

Replace the ignition control module. Refer to Ignition Control Module Replacement .

--

Go to Step 22

--

22
  1. Review and record Freeze Frame data.
  2. Clear DTCs.
  3. Start and idle the engine.
  4. Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame data).
  5. 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)?

--

Go to Step 2

System OK