DTC P0300 Engine Misfire Detected. Buick GL7 (Export) 2000

For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) has the ability to detect a misfire by monitoring the 3X reference from the ignition control (IC) module and camshaft position input signals from the camshaft position (CMP) sensor. The PCM monitors crankshaft speed variations to determine if a misfire is occurring.

If 2 percent or more of all cylinder firing events are misfires, the emission levels may exceed the mandated standards. The PCM determines the misfire level based upon 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 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause 3-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 in order to alert the vehicle operator of the potential of catalyst damage.

### Conditions for Running the DTC

• | No TP, MAP, ECT, CKP, CMP, MAF, or VSS DTCs are set.

• | The engine speed is between 500 and 5900 RPM.

• | The system voltage is between 9.0 and 18.0 volts.

• | The engine coolant temperature (ECT) indicates an engine temperature between 21°C (68°F) and 125°C (248°F).

• | The throttle angle is steady.

### Conditions for Setting the DTC

The PCM detects a crankshaft RPM variation indicating a misfire sufficient to cause 3-way catalytic converter damage or emissions levels to exceed the mandated standard.

### 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 History numbers 1 and 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 (CKP) sensor have recently been replaced, and the Crankshaft Position System Variation Learn 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 scan tool misfire counter values (Misfire History numbers 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, inspect for the following conditions:

• | 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 CKP sensor has been replaced.

Inspect for the following conditions:

• | Inspect the secondary ignition wires of the affected cylinder pair for disconnected ignition wires or for excessive resistance. The resistance should measure between 1 968 ohms per meter (600 ohms per foot).

• | A damaged or faulty ignition coil. Inspect for cracks, carbon tracking, or other damage. Also measure the coil secondary resistance. The secondary resistance should be between 5,000 - 8,000 ohms.

• | Substitute a known good coil. Swap ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

If the misfire is random, inspect for the following components:

• | The system grounds. Ensure all connections are clean and properly tightened.

• | The MAF sensor. A MAF sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.

• | The air induction system. Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Inspect for disconnected or damaged vacuum hoses, an incorrectly installed or faulty crankcase ventilation valve, or vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces.

• | Perform a fuel system pressure test. A faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.

• | The injectors. Perform injector coil/balance test in order to locate faulty injectors contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O-rings.

• | The EGR valve. Inspect for a leaking valve, adapter, or feed pipes which contribute to a lean condition or excessive EGR flow.

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

• | Loose terminal connection

- | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

- | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

• | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

• | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

### Test Description

The numbers below refer to 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 electronic brake control module (EBCM) may cause a misfire DTC to 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. The misfire current cylinder number 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. Depending on the cause of the misfire, the misfire history cylinder number counter will display a very large number for the misfiring cylinders. The values for the non-misfiring cylinders will be less than 1/2 as much as the misfiring cylinders. When investigating a misfire, always start with the items associated with the cylinders that has the largest number of counts stored in the misfire history cylinder number counter.

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

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

6. Inspect for the following conditions that may contribute to the engine misfire: the engine oil pressure, a damaged accessory drive belt or pulley, a damaged driven accessory, the generator, the water pump, the drive belt tensioner, and loose or broken motor mounts.

#### DTC P0300 Engine Misfire Detected

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Are any other DTCs set? | -- | Go to the applicable DTC table | Go to Step 3

3

1. Start and idle the engine.
2. Review and record the scan tool Freeze Frame data.
3. Operate the vehicle within the conditions present when the DTC set in the Freeze Frame data.
4. Monitor the scan tool Misfire Current Cylinder number display for each cylinder.

Is the Misfire Current number display incrementing for any cylinder?

--

Go to Step 5

Go to Step 4

4 | View the Misfire History Cylinder number display on the scan tool. Does the Misfire History Cylinder number 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

Perform a visual inspection of the following areas:

1. The vacuum hoses for improper connections and damage. Refer to Emission Hose Routing Diagram .
2. The spark plug wire connections at the coils and at the spark plugs.
3. The engine grounds and the PCM grounds. Ensure that the connections are in the proper locations, clean, and tight.
4. The EGR-to-intake manifold piping for proper connections and signs of damage

Did you find and correct the condition?

--

Go to Step 22

Go to Step 7

7 | Test the fuel pressure. Is the fuel pressure within the specified values? | 333-375 kPa (41-47 psi) | Go to Step 8 | Go to Fuel System Pressure Test

8

Test for proper fuel injector operation. Refer to 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) .

Did you find and correct the condition?

--

Go to Step 22

Go to Step 9

9

1. Visually inspect the ignition wires associated with the misfiring cylinders.

Important: : If carbon tracking is apparent at either end of the spark plug wires, replace the affected ignition wire and the associated ignition coil.

2. Visually inspect the ignition coils associated with the misfiring cylinders for the following conditions:

• | Carbon tracking, arcing, or damage

• | Connected to the incorrect cylinders at the coil and spark plug

• | Poor terminal connections at the coil and spark plug

Did you find and correct the condition?

--

Go to Step 22

Go to Step 10

10

1. Attach a J 26792 spark tester at the spark plug end of the ignition wire associated with the misfiring cylinders.
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 J 26792 spark tester.

Is a spark present?

--

Go to Step 15

Go to Step 11

11

1. Measure the resistance of the ignition wires associated with the cylinders that were indicated as misfiring.
2. Replace the ignition wires if the resistance measures more than the specified value.

Did you find and correct the condition?

1 968 ohms/m (600 ohms/ft)

Go to Step 22

Go to Step 12

12

1. Measure the resistance of the secondary ignition coils associated with the misfiring cylinders.
2. If the resistance is not between the specified range, replace the malfunctioning ignition coils.

Did you find and correct the condition?

5,000-8,000 ohms

Go to Step 22

Go to Step 13

13

1. Remove the ignition coils associated with the misfiring cylinders.
2. Inspect the coils for signs of carbon tracking, cracks, or other damage.
3. If a problem is found, replace the affected coils.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 14

14

1. Leave the ignition coils disconnected.
2. Connect a test lamp across the primary circuit terminals of the ignition module .

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

3. Remove the fuel injector fuse.
4. Observe the test lamp while cranking the engine.

Does the test lamp flash?

--

Go to Step 20

Go to Step 21

15

1. Remove the spark plugs from the cylinders associated with the misfire. Refer to Spark Plug Replacement in Engine Electrical.
2. Visually inspect the spark plug electrodes for excessive fouling.

Did you find and correct the condition?

--

Go to Base Engine Misfire Diagnosis in Engine Mechanical

Go to Step 16

16

Visually inspect the spark plugs for the following conditions:

Important: : If carbon tracking is apparent on any of the spark plugs, replace the affected spark plugs and the associated ignition wires. Refer to Spark Plug Replacement in Engine Electrical.

• | Carbon tracking, cracking, or other damage to the insulator

• | Electrode damage or an incorrect gap

Reinstall the spark plugs after completing the inspections.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 17

17 | Inspect for base engine mechanical problems. Refer to Base Engine Misfire Diagnosis in Engine Mechanical. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 18
18 | If the condition occurs while driving, check for transaxle TCC problems. Refer to Torque Converter Diagnosis in Automatic Transaxle-4T65-E. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 19
19 | Test for contaminants in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Did you find and correct the condition? | -- | Go to Step 22 | Go to Diagnostic Aids
20 | Replace the ignition coils associated with the misfiring cylinders. Did you complete the replacement? | -- | Go to Step 22 | --
21 | Replace the ignition control module. Did you complete the replacement? | -- | Go to Step 22 | --

22

1. Review and record the Freeze Frame data.
2. Clear the DTCs.
3. Start and idle the engine.
4. Operate the vehicle to within the conditions present when the DTC was set, as defined by the Freeze Frame data.
5. Monitor the scan tool Misfire Current Cylinder number display for each cylinder.

Is the Misfire Current Cylinder number display incrementing for any cylinder?

--

Go to Step 2

System OK