Circuit Description
The crankshaft position (CKP) sensor sends pulses to the powertrain control module (PCM) as the reluctor teeth rotate past the sensor. Besides using these pulses to synchronize ignition and fuel injector operation, the PCM also times the interval between each pulse and compares each new time interval with the previous one in order to determine when an excessive change in crankshaft speed has occurred. A misfire causes an unexpected change in crankshaft speed. A certain amount of crankshaft acceleration/deceleration is expected between each firing stroke, but if the crankshaft speed changes are morer than an expected amount, the PCM will interpret this as misfire. If 2 percent or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM monitors blocks of 200 engine revolutions and counts the number of misfires that occur in each block.
Conditions for Running the DTC
| • | The following sensor diagnostic trouble codes (DTCs) are not set: |
| - | Vehicle speed sensor (VSS) |
| - | Throttle position (TP) sensor |
| - | Manifold absolute pressure (MAP) sensor |
| - | Engine coolant temperature (ECT) sensor |
| - | CKP sensor |
| - | Camshaft position (CMP) sensor |
| - | Mass air flow (MAF) sensor |
| • | Engine speed between 550 and 5,850 RPM. |
| • | System voltage between 9 and 16 volts. |
| • | The ECT indicates an engine temperature between -6°C (+21°F) and +120°C (+248°F). |
| • | Throttle angle steady |
| • | Fuel level above 10 percent |
| • | DFCO and torque management not active |
Conditions for Setting the DTC
| • | Five out of 16 consecutive 200 revolution blocks containing 22 or more misfires is considered non- catalyst damaging misfire. |
| • | Any 200 revolution block containing an excessive amount of misfires, above a predetermined threshold, is considered catalyst damaging misfire. |
Action Taken When the DTC Sets
| • | If the misfire level is non-catalyst damaging, the PCM will illuminate the malfunction indicator lamp (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 torque converter clutch (TCC) operation. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data. |
| • | Any data previously stored in Freeze Frame will be overwritten when this DTC sets. The only exception to this is if a fuel trim DTC was already recorded in Freeze Frame. In this case, the fuel trim data stays in Freeze Frame and the Misfire Data will be stored in Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail within the same conditions that the DTC last failed. |
| Note that if the last failure was during a non-typical driving condition, the MIL may remain on longer than three drive trips. Review Freeze Frame for the last failure conditions. |
| • | 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
Notice: Use the connector test adapter kit J 35616-A for any test that
requires probing the following items:
• The PCM harness connectors • The electrical center fuse/relay cavities • The component terminals • The component harness connector
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 misfire counter values (Misfire Hist. #1 through #6) can be useful in determining whether the misfire affects a single cylinder or is random.
If the misfire is random, check for the following conditions:
| • | Incorrect CKP system variation data stored in the PCM -- Run the Crankshaft Position System Variation Learn . |
| • | 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 . |
| • | Contaminated fuel -- Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | EGR system -- Check for a 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 (HO2S). 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 Failure 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 HO2S can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s. |
| • | Running the vehicle low on fuel -- This DTC may set if the vehicle misfires while running out of fuel. If no misfire is currently present, but the History Counters indicate random misfire was once present, ask if the vehicle has been run out of fuel. If this is the case, no further repair may be necessary. |
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 Failure 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.
If the problem is intermittent, refer to Intermittent Conditions .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
Engine misfire can change engine load characteristics which may affect the TP sensor performance diagnostic. Correct any misfire before diagnosing DTC P0121. A malfunctioning injector circuit, crankshaft position system variation not learned, an intermittent CKP sensor problem, incorrect rough road data from the electronic brake control module (EBCM), etc. may cause a misfire DTC to be set. If any other DTCs are set with DTC P0300, except P0121, diagnose and repair the other DTC before using the DTC P0300 table.
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The Misfire Bar Graph, or 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.
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A misfiring or dead cylinder can sometimes affect the misfire counters for other cylinders. When performing this table, concentrating on the cylinder with the highest level of misfire and correcting the problem, may also correct the other cylinders that indicate misfire. This step splits the table between conditions that would cause a specific cylinder to misfire and conditions that would affect all cylinders, causing a random multiple cylinder misfire.
Step
| Action | Value(s) | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||||||
2 |
Important:: Incorrect CKP system variation data in the PCM may cause DTC P0300 to
set with no apparent misfire. Any of the following conditions can cause
incorrect CKP system variation data:
Has any service been performed recently that warrants running the CKP System Variation Learn procedure? | -- | ||||||||||||
Are any DTCs other than P0121 also set? | -- | Go to the applicable DTC table | ||||||||||||
Do any cylinders indicate an excessive amount of misfire? | -- | Go to Diagnostic Aids | ||||||||||||
Does the misfire appear to be affecting specific cylinders, not random cylinders? | -- | |||||||||||||
6 | Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections. Refer to Emission Hose Routing Diagram . Was problem found and corrected? | -- | ||||||||||||
7 | Visually and physically inspect the PCV valve for improper installation and for damaged O-rings. Refer to Crankcase Ventilation System Diagnosis . Was problem found and corrected? | -- | ||||||||||||
8 | Inspect the throttle body inlet and duct work for damage or for the presence of foreign objects that may alter the air flow sample through the MAF sensor. Was problem found and corrected? | -- | ||||||||||||
9 | Check the fuel pressure. Refer to Fuel System Pressure Test . Was problem found and corrected? | -- | ||||||||||||
10 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was a problem found and corrected? | -- | ||||||||||||
11 | Visually and physically inspect the PCM grounds and ignition control module grounds to ensure that they are clean, tight, and in their proper locations. Refer to Power Distribution Schematics in Wiring System. Was a problem found and corrected? | -- | ||||||||||||
12 | Visually and physically inspect the following areas for vacuum leaks:
Was a problem found and corrected? | -- | ||||||||||||
13 |
Was a problem found and corrected? | -- | ||||||||||||
14 | Check for proper ignition system operation. Refer to Secondary Ignition Diagnosis . Was a problem found and corrected? | -- | ||||||||||||
15 | Check for proper fuel injector operation. Refer to Fuel Injector Balance Test with Tech 2 . Was a problem found and corrected? | -- | ||||||||||||
16 | Check for vacuum leaks that may affect one cylinder more than others. Possible causes are:
Was a problem found and corrected? | -- | ||||||||||||
17 | Check for the following mechanical problems:
Was a problem found and corrected? | -- | ||||||||||||
18 |
Was a problem found and corrected? | -- | Go to Diagnostic Aids | |||||||||||
19 |
Is Misfire Current Cyl # display incrementing for any cylinder indicating a misfire currently occurring? | -- | ||||||||||||
20 | Review Captured Info using the scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
