GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 1997 🢒 Century 🢒 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 5 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
  1. No VSS, TP sensor, MAP sensor, ECT sensor, CKP sensor CMP sensor, or MAF sensor DTC(s) set.
  2. Engine speed between 550 and 5850 RPM.
  3. System voltage between 9 and 16 volts.
  4. The ECT indicates an engine temperature between -6°C (21°F) and 120°C (248°F).
  5. Throttle angle steady.

Conditions for Setting the DTC

The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause three-way catalytic converter damage or emissions levels to exceed mandated standard.

Action Taken When the DTC Sets
    • If the misfire level is non-catalyst damaging, the PCM will illuminate the MIL during the second key cycle in which the DTC sets.
    • If the misfire is severe enough to cause possible catalyst damage, the PCM will immediately flash the MIL while the misfire remains at catalyst damaging levels.
    • The PCM will disable TCC operation.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
    • The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been 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 the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids

The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as DTC status Failed Since Code Clear, the misfire history counters (Misfire Hist #1 - #6) will still contain a value that represents the level of misfire detected on each cylinder. The scan tool displayed misfire counter values (Misfire Hist. #1 through #6) can be useful in determining whether the misfire affects a single cylinder, a cylinder pair (cylinders that share an ignition coil-1/4, 2/5, 3/6), or is random. If the largest amount of activity is isolated to a cylinder pair, 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. The wires should measure under 7,000 ohms (7 K ohms).
    • Damaged Or Malfunctioning Ignition Coil.
        Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be between 5000 ohms and 8000 ohms (5K ohms and 8K ohms).
    • Substitute a Known Good Coil.
        Switch ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

If the misfire is random, check for the following conditions

    • System Grounds.
        Ensure all connections are clean and properly tightened.
    • 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 with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the Mass Airflow Sensor Replacement .
    • Damaged accessory drive belt or driven accessory.
        A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.
    • 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 .
        Refer to Fuel System Pressure Test .
    • Fuel injector(s) . Refer to Fuel Injector Solenoid Coil Test or Fuel Injector Solenoid Coil Test .
    • Contaminated Fuel. Refer to Alcohol/Contaminants-in-Fuel Test .
    • 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 below 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 g/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.

Test Description

Number(s) below refer to the step number(s) 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. Steps 5 through 12 check for conditions that can cause a random cylinder misfire.

    5.

  5. Steps 13 through 23 check for conditions that can cause a nonrandom or single cylinder misfire.

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 the Powertrain On Board Diagnostic (OBD) System Check

2

Are any of the following DTCs also set?

--

Go to the other DTC(s) first

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 4

Refer to Diagnostic Aids

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 13

5

  1. Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections. Refer to Emission Hose Routing Diagram .
  2. If a problem is found, repair as necessary.

Was problem found?

--

Go to Step 25

Go to Step 6

6

  1. Visually and physically inspect the PCV valve for improper installation and for damaged O-rings. Refer to Positive Crankcase Ventilation Valve .
  2. If a problem is found, repair as necessary.

Was problem found?

--

Go to Step 25

Go to Step 7

7

  1. Inspect the throttle body inlet screen for damage or for the presence of foreign objects that may alter the air flow sample through the MAF sensor.
  2. If a problem is found, repair as necessary.

    3. Refer to Throttle Body Assembly Replacement .

Was problem found?

--

Go to Step 25

Go to Step 8

8

  1. Check fuel pressure. Refer to Fuel System Pressure Test .
  2. If a problem is found, repair as necessary.

Was problem found?

--

Go to Step 25

Go to Step 9

9

  1. Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Test .
  2. If a problem was found, repair as necessary.

Was a problem found?

--

Go to Step 25

Go to Step 10

10

  1. Visually and physically inspect the PCM injector grounds, power grounds and sensor grounds to ensure that they are clean, tight, and in their proper locations. Refer to Ground Distribution in Electrical Diagnosis.
  2. If a problem is found, repair as necessary. Refer to Repair Procedures in Electrical Diagnosis.

Was a problem found?

--

Go to Step 25

Go to Step 11

11

  1. Visually and physically inspect the following areas for vacuum leaks:
    2. • Intake manifold.
    • EGR adapter.
    • EGR valve.
    • EGR feed pipes.
    • Injector O-rings.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 25

Go to Step 12

12

  1. Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .
  2. Visually/physically inspect the valve to ensure that the pintle is not sticking partially open. Also, inspect the EGR valve pintle and seat for carbon deposits or burrs that may interfere with the pintle closing completely.
  3. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 25

Go to Step 13

13

  1. Check for proper fuel injector operation. Refer to the Fuel Injector Solenoid Coil Test .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 25

Go to Step 14

14

  1. Visually and physically inspect the ignition wires associated with the cylinder(s) which were misfiring to ensure that they are not damaged and are connected to the proper cylinders at the coils and at the spark plugs.
  2. If a problem is found, repair as necessary. Refer to Secondary Wiring in Ignition System.

Was a problem found?

--

Go to Step 25

Go to Step 15

15

  1. Install a J 26792 spark tester at the spark plug end of the ignition wire for the cylinder that is indicated by the Misfire Current Cyl # counters or Misfire History Cyl # counters as having the most severe misfire (largest number of counts).
  2. Jumper the spark plug end of the companion cylinder ignition wire to engine ground.

    3. 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. A crisp, blue spark should be observed.

Is adequate spark present?

--

Go to Step 20

Go to Step 16

16

  1. Remove and visually/physically inspect the ignition wires associated with the cylinders that were indicated as misfiring. Ensure that the wires and boots are free of carbon tracking and the insulation is not damaged.
  2. If a problem is found, replace affected ignition wire(s) as necessary. Refer to Secondary Wiring in Ignition System.

    3. Important: If carbon tracking or terminal discoloration is apparent at the ignition coil end of any of the ignition wires, replace the affected ignition wire and the associated ignition coil. Refer to Ignition Control Module Replacement .

Was a problem found?

--

Go to Step 25

Go to Step 17

17

  1. Measure the resistance of the ignition wires associated with the cylinders that were indicated as misfiring.
  2. Replace any ignition wire(s) that measure greater than the specified value. Refer to Secondary Wiring in Ignition System.

Was a problem found?

7K ohms (7,000 ohms)

Go to Step 25

Go to Step 18

18

  1. Remove and visually/physically inspect the ignition coil(s) associated with the cylinders that were indicated as misfiring. Ensure that the coil(s) and coil towers are free of cracks and carbon tracking.
  2. If a problem is found, replace damaged ignition coil(s) as necessary. Refer to Ignition Control Module Replacement .

Was a problem found?

--

Go to Step 25

Go to Step 19

19

  1. Measure the ignition coil secondary resistance.
  2. If resistance is not between the specified values, replace the malfunctioning ignition coil(s) as necessary. Refer to Ignition Control Module Replacement .

Was a problem found?

5K-8K ohms

(5000-8000 ohms)

Go to Step 25

Go to Step 24

20

  1. Remove the spark plugs from the cylinders that were indicated as misfiring. Go to Spark Plugs in Ignition System.
  2. Visually inspect the spark plug electrodes for excessive fouling. Refer to Spark Plugs in Ignition System.

Was a problem found?

--

Go to Engine Misfire Mechanical Diagnosis in Engine General Information

Go to Step 21

21

  1. Visually inspect spark plug insulators for cracks, carbon tracking, or other damage.
  2. Check spark plug electrodes for incorrect gap.
  3. If a problem is found, replace affected spark plug(s) as necessary. Refer to Spark Plugs in Ignition System.

Was problem found?

--

Go to Step 25

Go to Step 22

22

  1. Check for an engine mechanical problem.
    2. • Damaged accessory drive belt or pulley.
    • Damaged driven accessory (generator, water pump, drive belt tensioner, ect).
    • Base engine mechanical problem. Refer to Engine Misfire Mechanical Diagnosis in Engine General Information.
    • Loose or broken motor mount.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 25

Go to Step 23

23
  1. Check for a transaxle TCC problem.
    2. • Refer to 4T60-E Automatic Transaxle Diagnosis (VIN K) .
    • Refer to 4T65-E Automatic Transaxle Diagnosis (VIN 1) .
  2. If a problem is found, repair the transaxle as necessary. Refer to 4T65-E Automatic Transaxle Unit Repair .
    3. • Refer to 4T60-E Automatic Transaxle Unit Repair (VIN K) .
    • Refer to 4T65-E Automatic Transaxle Unit Repair (VIN 1) .

Was a problem found?

--

Go to Step 25

Refer to Diagnostic Aids

24

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

Is action complete?

--

Go to Step 25

--

25
  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