Refer to Engine Controls Schematics
Cell 21: VCM Power and Grounding, MIL, DLC
.
Circuit Description
The crankshaft position (CKP) sensor is the primary input to determine if misfire is occurring. Engine misfire is detected by monitoring crankshaft speed variations between cylinders. If a crankshaft deceleration occurs during a combustion or power stroke, the control module will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is more than a maximum allowable speed, the misfire is detected. Misfire may occur in a specific cylinder or in all cylinders randomly.
When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP. The control module determines which cylinder has misfired based upon the camshaft position (CMP) sensor input. Misfire data is stored for each cylinder in separate accumulators. After 100 combustion events, the misfire totals are compared to a calibrated maximum number. If the misfire is excessive, this diagnostic trouble code (DTC) will set.
Conditions for Running the DTC
Important: If start-up ECT is below -7°C (20°F), misfire detection is delayed until ECT is greater than 21°C (70°F). If start-up ECT is greater than -7°C (20°F), misfire detection begins after a 5 second delay.
| • | No active VS sensor DTCs |
| • | No active TP sensor DTCs |
| • | No active MAF sensor DTCs |
| • | No active CKP sensor DTCs |
| • | No active CMP sensor DTCs |
| • | The fuel level is greater than 10% |
| • | The engine speed is between 450 and 5,000 RPM |
| • | The system voltage is between 11 and 16 volts |
| • | The throttle position is steady within 2% for 100 ms |
Conditions for Setting the DTC
The VCM detects a deceleration in the crankshaft speed characteristic of either an emission type misfire or a catalyst damaging type misfire.
Action Taken When the DTC Sets
If the VCM determines that the engine misfire is significant enough to have a negative impact on emissions, the VCM turns ON the malfunction indicator lamp (MIL) after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.
Conditions for Clearing the MIL or DTC
| • | The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed. |
| • | A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle. |
| • | Use a scan tool in order to clear the DTCs. |
Diagnostic Aids
The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring or use the history misfire counter for misfires that are not currently present.
Many different conditions could cause an intermittent misfire.
Check for the following conditions:
| • | Check the IC control circuit for an intermittent short to ground. |
| • | Check the spark plug wires and the coil wire for the following conditions: |
| - | Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap. |
| - | Check the wire routing in order to ensure that crossfiring is not occurring. |
| - | If the misfire occurs when the weather is damp, the problem could be due to worn plug wires. |
| - | In order to test for this condition, spray the wires with water and with the engine running, watch for spark to jump from the wires. If a spark is visible, replace the wires. |
| • | Check for contaminated and a low fuel level and the following conditions: |
| • | Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition. |
| • | If the fuel level is low, contaminants in the bottom of the fuel tank could enter into the fuel metering system. |
Sticking intake or exhaust valves on engines with a misfire when cold.
Check HO2S for abnormal voltage readings.
Check for a vacuum leak as a possible cause of the engine misfire.
For more information, refer to Fuel System Diagnosis
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside the insulation.
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
If a repair is necessary, then refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
The misfire is considered random on all cylinders if, while viewing the misfire counters in the misfire data list, the misfire seems to move to different cylinders.
-
The misfire is considered consistent if the misfire is occurring on the same cylinder(s) consistently.
-
When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, record the Freeze Frame and the Failure records for reference with a scan tool. This data will be lost when you use the Clear DTC Information function. Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 | Install the scan tool. Are there any other DTCs stored? | -- | Go to the applicable DTC table | |
3 |
Is the Misfire counter increasing? | -- | ||
4 |
Does the scan tool indicate a misfire is present? | -- | ||
Is the misfire random an all cylinders? | -- | |||
Is the misfire consistent on specific cylinders? | -- | -- | ||
Does the tester indicate spark is consistent on all tested ignition wires? | -- | |||
8 |
Did you find a problem? | -- | ||
9 |
Did the misfire(s) change cylinder(s)? | -- | ||
10 | Perform the fuel system diagnostic. Refer to Fuel System Diagnosis Repair as necessary. Did you find a problem? | -- | ||
11 | Perform the Fuel Injector Balance test. Refer to Fuel Injector Balance Test with Tech 2 Did you find a problem? | -- | ||
12 |
Does the injector test lamp flash for each injector? | -- | ||
13 |
Was a Repair made? | -- | ||
14 | Replace the spark plugs. Refer to Spark Plug Replacement in Engine Electrical. Is the action complete? | -- | -- | |
15 | Physically and visually check for these conditions in the following order:
Repair as necessary Did you find a problem? | -- | ||
16 |
Did you find a problem? | -- | Go to Diagnostic Aids | |
17 | Perform the CKP Variation Learn procedure. Refer to Crankshaft Position System Variation Learn Is the action complete? | -- | -- | |
18 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
19 | Does the scan tool display any additional undiagnosed DTC's? | -- | Go to the applicable DTC table | System OK |
