DTC P0300 Engine Misfire Detected. Buick Regal (China) (CNG) 1999

For 1990-2009 cars only

Makes 🢒 Buick 🢒 1999 🢒 Regal (China) (CNG) 🢒 Engine 🢒 Engine Controls - 3.0L (L46) 🢒 DTC P0300 Engine Misfire Detected

Engine Controls Components
CMP, CKP and ICM
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The 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 (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 TP, MAP, ECT, CKP, CMP, MAF sensor, VSS, DTCs set.

•	Engine speed between 550 and 5900 RPM.

•	System voltage between 9.0 and 18.0 volts.

•	The ECT indicates an engine temperature between 20°C (68°F) and 120°C (248°F).

•	Throttle angle steady.

Conditions for Setting the DTC

The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause catalytic converter damage IF EQUIPPED.

Action Taken When the DTC Sets

•	The PCM will not illuminate the malfunction indicator lamp (MIL).

•	The PCM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

•	A 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.

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 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 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:

•	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.

•	Secondary Ignition Wires: Check wires for affected cylinder pair for disconnected ignition wires or for excessive resistance (the wires should measure should be 600ohms per foot (1 968ohms per meter).

•	Damaged Or Faulty Ignition Coil: Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be between 5000ohms and 8000ohms (5Kohms and 8Kohms).

•	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, check for the following conditions:

•	System Grounds: Ensure all connections are clean and properly tightened.

•	MAF: A Mass Air Flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.

•

Air Induction System: Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty 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 faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.

•	Injector(s): Perform injector coil/balance test to locate faulty injector(s) contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O rings.

•	EGR: Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

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:

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.

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.

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.

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

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

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

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

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Are any other DTCs set?

Go to applicable DTC

Go to Step 3

Start and idle the engine.
Review and record scan tool Freeze Frame data.
Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame data).
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

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

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

Perform a visual inspection found in Symptoms of the following areas:

•	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 Power and Grounding Component Views

If a problem is found, repair as necessary.

Was a problem found?

Go to Step 22

Go to Step 7

Check the fuel pressure.

Is the fuel pressure within the specified values?

333-375 kPa (41-47 psi)

Go to Step 8

Go to the Fuel System Pressure Test

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) .
If a problem is found, repair as necessary.

Was a problem found?

Go to Step 22

Go to Step 9

Visually inspect the ignition wire(s) and coil(s) associated with the misfiring cylinder(s) for the following condition(s):

•	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.

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.

Was a problem found?

Go to Step 22

Go to Step 10

install J 26792 Spark Tester at the spark plug end of the ignition wire associated with the misfiring cylinder(s).
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).
Crank the engine while observing the J 26792 spark tester.

Is spark present?

Go to Step 15

Go to Step 11

Measure the resistance of the ignition wire(s) associated with the cylinder(s) that were indicated as misfiring.
Replace the ignition wire(s) if the resistance measures greater than the specified value.

Was a problem found?

600ohms per foot(1 968ohms per meter)

Go to Step 22

Go to Step 12

Measure the resistance of the secondary ignition coil(s) associated with the misfiring cylinder(s).
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-8Kohms (5,000-8,000ohms)

Go to Step 22

Go to Step 13

Remove the ignition coil(s) associated with the misfiring cylinder(s).
Inspect the coil(s) for signs of carbon tracking, cracks or other damage.
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

Leave the ignition coil(s) disconnected.
Connect a J 35616-200 test light across the ignition module primary circuit terminals.
Remove the fuel injector fuse.
Observe the J 35616-200 test light while cranking the engine.

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

Does the J 35616-200 test light blink?

Go to Step 20

Go to Step 21

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

Was a problem found?

Go to Base Engine Misfire Diagnosis

Go to Step 16

Visually inspect the spark plugs for the following condition(s):

•	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).

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

Reinstall the spark plug.Refer to Spark Plug Replacement .

Check for engine mechanical problems.
Base engine mechanical problems.Refer to Base Engine Misfire Diagnosis .
If a problem is found, repair as necessary.

Was a problem found?

Go to Step 22

Go to Step 18

If condition occurs while driving, check for transaxle TCC problems. Refer to Torque Converter Diagnosis .
If a problem is found, repair as necessary.

Was a problem found?

Go to Step 22

Go to Step 19

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

Was a problem found?

Go to Step 22

Refer to Diagnostic Aids

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

Is the action complete?

Go to Step 22

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

Go to Step 22

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