Refer to
Cell 21: Ignition Controls
at beginning of this section.
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 16.0 volts. |
| • | The ECT indicates an engine temperature between -6°C (21°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 three-way catalytic converter damage or emissions levels to exceed mandated standard.
Action Taken When the DTC Sets
| • | The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | 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 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 Powertrain Control Module Replacement/Programming has not been performed. If the diagnostic table does not identify a problem then perform the Powertrain Control Module Replacement/Programming . 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:
| • | Secondary Ignition Wires: Check wires for affected cylinder pair for disconnected ignition wires or for excessive resistance 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 (the wires should measure under 8Kohms (8000ohms). |
| • | 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. Refer to Power and Grounding Component Views . |
| • | 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. Refer to 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. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) , Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) , Fuel Injector Balance Test . 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
The numbers below refer to the step numbers 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 cylinders; values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinders. When investigating a misfire, always start with items associated with the cylinders 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 mounts.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Was Powertrain OBD System Check performed? | -- | ||||||||||
Are any other DTCs set? | -- | Go To Applicable DTCs | ||||||||||
Is the Misfire Current # display incrementing for any cylinder (indicating a misfire currently occurring)? | -- | |||||||||||
View the Misfire History Cyl # display on the scan tool. Does Misfire History Cyl # display a very large value for more than one cylinder? | -- | |||||||||||
Are the misfire values displayed on the Misfire History Cyl # related to companion cylinders (i.e 1/4, 2/5, 3/6)? | -- | |||||||||||
6 |
Was a problem found? | -- | ||||||||||
7 | Check the fuel pressure. Is the fuel pressure within the specified values? | 333-375 kPa (41-47 psi) | Go to the Fuel System Pressure Test | |||||||||
8 |
Was a problem found? | -- | ||||||||||
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? | -- | |||||||||||
10 |
Is spark present? | -- | ||||||||||
11 |
Was a problem found? | 600ohms per foot (1 968ohms per foot | ||||||||||
12 |
Was a problem found? | 5K-8Kohms (5,000-8,000ohms) | ||||||||||
13 |
Was a problem found? | -- | ||||||||||
14 |
Does the test lamp blink? | -- | ||||||||||
15 |
Was a problem found? | -- | Go to Base Engine Misfire Diagnosis in Engine Mechanical | |||||||||
16 |
Was a problem found? | -- | ||||||||||
Reinstall the spark plug. Refer to Spark Plug Replacement in Engine Electrical.
Was a problem found? | -- | |||||||||||
18 |
Was a problem found? | -- | ||||||||||
19 |
Was a problem found? | -- | Go to Diagnostic Aids | |||||||||
20 |
Important: : Be sure to reinstall the fuel injector fuse. Replace the ignition coils associated with the misfiring cylinders. Refer to Ignition Coil(s) Replacement . Is the action complete? | -- | -- | |||||||||
21 |
Important: : Be sure to reinstall the fuel injector fuse. Replace the ignition control module. Refer to Ignition Control Module Replacement | -- | -- | |||||||||
22 |
Is Misfire Current Cyl # display incrementing for any cylinder (indicating a misfire currently occurring)? | -- | System OK |
