System Description
The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensors and the camshaft (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause catalytic converter damage. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present.
Conditions for Running the DTC
Important: If the start-up ECT is less than 7°C (20°F), misfire detection is delayed until 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.
| • | DTCs P0101, P0102, P0103, P0117, P0118, P0128, P0335, P0336, P0341, P0343, P0500, P0502, P0503, P1114, P1115, P1121, P1122, P1220, P1258, P1336, or P1345 are not set. |
| • | The fuel level is more than 10 percent |
| • | The engine speed is between 450-5,000 RPM. |
| • | The system voltage is between 11-16 volts. |
| • | The throttle position (TP) is steady within 2 percent for 100 ms. |
Conditions for Setting the DTC
The powertrain control module (PCM) detects a deceleration in the crankshaft speed that is characteristic of either an emission type misfire or of a catalyst-damaging type misfire.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Diagnostic Aids
The misfire index counts the number of misfires. The scan tool can monitor the misfire index. There is a current and a 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 conditions could cause an intermittent misfire. Check the following conditions:
| • | Check the ignition control (IC) circuit for an intermittent short to ground. |
| • | Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap. |
| • | Ensure that the coil wire is securely attached to the distributor cap and the coil. |
| • | Check the wire routing to ensure that cross-firing is not occurring. |
| • | If the misfire occurs when the weather is damp, the problem could be due to worn plug wires. |
| • | Check the fuel for the following conditions: |
| - | Dirty or contaminated fuel could cause a misfire condition. |
| - | If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and a possible misfire condition. Check the fuel trim number 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 can also cause a misfire. |
| • | Check for sticking intake or exhaust valves on engines which misfire when the engine is cold. |
| • | Check the heated oxygen sensor (HO2S) for abnormal voltage readings. |
| • | Check for a vacuum leak as a possible cause of the engine misfire. |
An intermittent may be caused by any of the following conditions:
| • | A poor connection |
| • | Rubbed through wire insulation |
| • | A broken wire inside the insulation |
Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. If a repair is necessary, refer to Wiring Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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If the Misfire Current Counters are incrementing, but the engine is NOT misfiring, this indicates a mechanical condition. For example, an accessory drive belt could cause this condition.
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The spark should be consistent when you test the spark at the spark plug wires. The presence of few sparks then nothing indicates no spark.
Step | Action | Values | Yes | No | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||||||||||||||||
2 |
Are there any other DTCs set? | -- | Go to Step 3 | |||||||||||||||||||||||
3 |
Are any of the Misfire Current Counters incrementing? | -- | Go to Step 5 | Go to Step 4 | ||||||||||||||||||||||
4 |
Does the DTC fail this ignition cycle? | -- | Go to Step 5 | Go to Diagnostic Aids | ||||||||||||||||||||||
Is the engine misfiring? | -- | Go to Step 6 | Go to Symptoms - Engine Mechanical -4.3L | |||||||||||||||||||||||
6 | Is there only one Misfire Current Counter incrementing? | -- | Go to Step 7 | Go to Step 13 | ||||||||||||||||||||||
Does the spark tester spark and is the spark consistent? | -- | Go to Step 9 | Go to Step 8 | |||||||||||||||||||||||
8 |
Is the resistance less than the specified value? | 625 ohms Per 30 CM (Per Foot) | Go to Step 16 | Go to Step 15 | ||||||||||||||||||||||
9 |
Important: If the spark plug is gas, coolant, or oil fouled, determine the cause of the fouling. Refer to DTC P0172 or P0175 for spark plugs that are gas fouled. Refer to Symptoms - Engine Mechanical -4.3L for spark plugs that are coolant or oil fouled. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 10 | ||||||||||||||||||||||
10 |
Did the misfire move with the spark plug? | -- | Go to Step 14 | Go to Step 11 | ||||||||||||||||||||||
11 |
Important:
Perform the Fuel Injector Coil Test. Refer to Fuel Injector Solenoid Coil Test . Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 12 | ||||||||||||||||||||||
12 | Perform the Fuel System Diagnosis. Refer to Fuel System Diagnosis . Did you find and correct the condition? | -- | Go to Step 17 | Go to Symptoms - Engine Mechanical -4.3L | ||||||||||||||||||||||
13 | Inspect or test for the following conditions:
Did you find and correct the condition? | -- | Go to Step 17 | |||||||||||||||||||||||
14 | Replace the spark plugs. Refer to Spark Plug Replacement . Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||||||||||||||||||
15 | Replace the spark plug wires. Refer to Spark Plug Wire Replacement . Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||||||||||||||||||
16 | Replace the distributor cap. Refer to Distributor Overhaul . Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||||||||||||||||||
17 |
Does the DTC run and pass? | -- | Go to Step 18 | Go to Step 2 | ||||||||||||||||||||||
18 | With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |
