Refer to Engine Controls Schematics
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 the start-up engine coolant temperature (ECT) is less than -7°C (20°F), misfire detection is delayed until the ECT is more than 21°C (70°F). If the start-up ECT is more than -7°C (20°F), misfire detection begins after a 5-second delay.
| • | No VS sensor DTCs are active. |
| • | No TP sensor DTCs are active. |
| • | No MAF sensor DTCs are active. |
| • | No CKP sensor DTCs are active. |
| • | No CMP sensor DTCs are active. |
| • | The fuel level is more than 10 percent. |
| • | The engine speed is between 450-5000 RPM. |
| • | The system voltage is between 11-16 volts. |
| • | The throttle position is steady within 2 percent 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 counter and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring. Use the history misfire counter for misfires that are not currently present.
Many different condition could cause an intermittent misfire.
| • | Check the IC control circuit for a intermittent short to ground |
| • | Check the spark plug wires and the coil wire: |
| - | Ensure that the spark plug wires are securely attached to the spark plugs and to the distributor cap. |
| - | Ensure that cross-firing is not occurring because of faulty wire routing. |
| - | Check for worn spark plug wires if the misfire occurs when the weather is damp. |
| • | Check the fuel for: |
| - | The fuel condition and quality-Dirty or contaminated fuel could cause a misfire condition. |
| - | A low fuel level-The fuel pump may draw air into the fuel rail, causing a stumble and a possible misfire condition. Check the fuel trim numbers in the freeze frame in order to determine if this has occurred. This condition would be likely if the short term fuel number was more than +20 |
| - | A restricted fuel filter-Restrictions can cause misfire conditions. |
| • | Check the HO2S for abnormal voltage readings. |
| • | Check for a vacuum leak as a possible cause of the engine misfire. |
| • | Check for sticking intake or exhaust valves can cause a misfire when the engine is cold. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The misfire is considered random on all cylinders if it seems to move to different cylinders while you view the counters in the misfire data list.
-
The misfire is considered consistent if the misfire occurs on the same cylinders consistently.
-
When you check 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 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||||||||||
2 | Install the scan tool. Are any other DTCs stored? | -- | Go to the applicable DTC | |||||||||||||||
3 |
Is the misfire counter increasing? | -- | ||||||||||||||||
4 |
Does the scan tool indicate a misfire is present? | -- | ||||||||||||||||
Is the misfire random on all cylinders? | -- | |||||||||||||||||
Is the misfire consistent on specific cylinders? | -- | -- | ||||||||||||||||
Does the tester indicate that spark is consistent on all of the tested ignition wires? | -- | |||||||||||||||||
8 |
Did you find a problem? | -- | ||||||||||||||||
9 |
Did the misfires change cylinders? | -- | ||||||||||||||||
10 |
Did you find a problem? | -- | ||||||||||||||||
11 |
Does the lamp flash on all tested circuits while the engine is cranked? | 10 sec | ||||||||||||||||
12 | Perform the Fuel Injector Balance test. Refer to Fuel Injector Balance Test with Tech 2 . Did you find a problem? | -- | ||||||||||||||||
13 | Replace the spark plugs. Refer to Spark Plug Replacement in Engine Electrical. Is the action complete? | -- | -- | |||||||||||||||
14 | Check for:
Repair as necessary. Did you find a problem? | -- | -- | |||||||||||||||
15 |
Did you find a problem? | -- | Go to Diagnostic Aids | |||||||||||||||
16 | Perform the CKP Variation Learn Procedure. Refer to Crankshaft Position System Variation Learn . Is the action complete? | -- | -- | |||||||||||||||
17 |
Does the scan tool indicate the diagnostic Passed? | -- | ||||||||||||||||
18 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to applicable DTC | System OK |
