If the misfire is frequent enough to trigger detection of increased emissions during two consecutive driving cycles, the MIL will come on, and DTC P0300 (and some combination of P0301 through P0304) will be stored.
If the misfire is frequent enough to damage the catalyst, the MIL will flash whenever the misfire occurs, and DTC P0300 (and some combination of P0301 through P0304) will be stored. When the misfire stops, the MIL will remain on.
Troubleshoot the following DTCs first, if any of them were stored along with the random misfire DTC(s) (Because parts can sometimes fail without setting DTC's you should also do a physical inspection of the systems listed below):
Are the misfire counts increasing on any cylinder at idle?
YES
The engine is misfiring at idle, go to step 2.
NO
The engine does not misfire at idle, go to step 3.
2.
Determine Affected Cylinders
?1.
Continue to monitor the misfire parameters in the DATA LIST.
Signal
Current conditions
Values
Unit
Cyl1 Misfire B
Cyl2 Misfire B
Cyl3 Misfire B
Cyl4 Misfire B
Are the misfire counts generally isolated to a single cylinder?
YES
The misfire is isolated to a single cylinder.
If the misfire counts are increasing every engine revolution (completely inactive cylinder), go to Single Cylinder Continuous Misfire at Idle troubleshooting on step 6.
If the misfire counts are increasing every few engine revolutions (partially inactive cylinder), go to Single Cylinder Non-Continuous Misfire at Idle troubleshooting on step 20.
NO
The misfire affects multiple cylinders.
If the misfire counts are increasing every engine revolution (completely inactive cylinder), go to Multiple Cylinder Continuous Misfire at Idle troubleshooting on step 29.
If the misfire counts are increasing every few engine revolutions (partially inactive cylinder), go to Multiple Cylinder Non-Continuous Misfire at Idle troubleshooting on step 46.
3.
Problem verification - Static Load
?1.
Increase the engine speed to 3,500 rpm.
?2.
Hold the engine speed steady, and monitor the misfire parameters in the DATA LIST.
Signal
Current conditions
Values
Unit
Cyl1 Misfire B
Cyl2 Misfire B
Cyl3 Misfire B
Cyl4 Misfire B
Are the misfire counts increasing on any cylinder?
YES
The engine is misfiring under load. Go to step 4.
NO
The engine does not misfire under a static load. Go to step 5.
4.
Determine Affected Cylinders
?1.
Continue to monitor the misfire parameters in the DATA LIST.
Signal
Current conditions
Values
Unit
Cyl1 Misfire B
Cyl2 Misfire B
Cyl3 Misfire B
Cyl4 Misfire B
Are misfire counts generally isolated to a single cylinder?
YES
The misfire is isolated to a single cylinder, go to Single Cylinder Misfire. Under Load troubleshooting on step 54.
NO
The misfire affects multiple cylinders, go to Multiple Cylinder Misfire. Under Load troubleshooting on step 59.
5.
Problem verification - On-board Snapshot
?1.
Open the ONBOARD SNAPSHOT with the HDS.
?2.
Scroll through the snapshot, and find when the MISFIRE value begins to increase.
?3.
Record the value of these parameters as the MISFIRE value increases:
Signal
Values
Unit
ECT SENSOR 1
°F
ENGINE SPEED
RPM
VEHICLE SPEED
MPH
MAP SENSOR
V
TP SENSOR
V
?4.
Drive the vehicle according to the conditions you identified above while monitoring the misfire counts.
Signal
Current conditions
Values
Unit
ECT SENSOR 1
ENGINE SPEED
VEHICLE SPEED
MAP SENSOR
TP SENSOR
Signal
Current conditions
Values
Unit
Cyl1 Misfire B
Cyl2 Misfire B
Cyl3 Misfire B
Cyl4 Misfire B
Are the misfire counts increasing on any cylinder?
YES
The engine is misfiring under the specific operating conditions above.
If the misfire occurred when the engine was cold, go to Misfire When Cold troubleshooting on step 64.
If the misfire occurred when the engine was loaded, go to step 4.
NO
Intermittent failure; the engine is not misfiring at this time.?
NOTE: If the misfire cannot be reproduced, there is a possibility that the problem may have been caused by poor quality fuel. The problem may have disappeared after refueling with fresh fuel. The MIL DIST (MIL Distance) value in the Data List in combination with the fuel level may help you make this determination.
Record your test results in the chart shown below.
Leakage (%)
Leakage Area
No. 1
No. 2
No. 3
No. 4
Does any cylinder have leakage greater than 10 %?
YES
If the majority of the air is escaping from exhaust pipe or throttle body, check for tight valve clearance and adjust valve clearance to specification (cold), then retest. Determine the cause of the cylinder leakage and repair the engine as needed.?
NO
The cylinders are sealing properly. Go to step 19.
NOTE: ECT Sensor 1 must read less than 100 °F (38 °C) before adjusting the valve clearance. Shortcutting this step may cause incorrect valve clearance and engine misfire.
?2.
Recheck the MAP SENSOR fluctuation at idle.
Does the MAP SENSOR continuously fluctuate 0.04 V or more at idle, no loads, engine hot?
YES
Adjusting the valve clearance did not correct the excessive MAP sensor fluctuation. Go to step 26.
NO
The excessive MAP sensor fluctuation was caused by incorrect valve clearance.?
Record your test results in the chart shown below.
Leakage (%)
Leakage Area
No. 1
No. 2
No. 3
No. 4
Does any cylinder have leakage greater than 10 %?
YES
If the majority of the air is escaping from exhaust pipe or throttle body, check for tight valve clearance and adjust each cylinder as needed, then retest. Determine the cause of the cylinder leakage and repair the engine as needed.?
NO
The cylinders are sealing properly. Go to step 28.
Drive the vehicle under the conditions when the misfire was duplicated.
?3.
Monitor the misfire parameters in the DATA LIST.
Signal
Current conditions
Values
Unit
Cyl1 Misfire B
Cyl2 Misfire B
Cyl3 Misfire B
Cyl4 Misfire B
Do the misfire counts stay on the problem cylinder when the fuel injector is swapped?
YES
The fuel injector is OK. All of the failures have been checked for this type of misfire. Go to step 1 in the Problem verification procedure and ensure you selected the correct troubleshooting procedure.
NOTE: ECT Sensor 1 must read less than 100 °F (38 °C) before adjusting the valve clearance. Shortcutting this step may cause incorrect valve clearance and engine misfire.
?2.
Recheck the MAP SENSOR fluctuation at idle.
Does the MAP SENSOR continuously fluctuate 0.04 V or more at idle, no loads, engine hot?
YES
Adjusting the valve clearance did not correct the excessive MAP sensor fluctuation. Go to step 43.
NO
The excessive MAP sensor fluctuation was caused by incorrect valve clearance.?
Record your test results in the chart shown below.
Leakage (%)
Leakage Area
No. 1
No. 2
No. 3
No. 4
Does any cylinder have leakage greater than 10 %?
YES
If the majority of the air is escaping from exhaust pipe or throttle body, check for tight valve clearance and adjust each cylinder as needed, then retest. Determine the cause of the cylinder leakage and repair the engine as needed.?
NO
The cylinders are sealing properly. Go to step 45.
NOTE: ECT Sensor 1 must read less than 100 °F (38 °C) before adjusting the valve clearance. Shortcutting this step may cause incorrect valve clearance and engine misfire.
?2.
Recheck the MAP SENSOR fluctuation at idle.
Does the MAP SENSOR continuously fluctuate 0.04 V or more at idle, no loads, engine hot?
YES
Adjusting the valve clearance did not correct the excessive MAP sensor fluctuation. Go to step 49.
NO
The excessive MAP sensor fluctuation was caused by incorrect valve clearance.?
Record your test results in the chart shown below.
Leakage (%)
Leakage Area
No. 1
No. 2
No. 3
No. 4
Does any cylinder have leakage greater than 10 %?
YES
If the majority of the air is escaping from exhaust pipe or throttle body, check for tight valve clearance and adjust each cylinder as needed, then retest. Determine the cause of the cylinder leakage and repair the engine as needed.?
NO
The cylinders are sealing properly. Go to step 51.
Inspect for evidence of fuel contamination such as diesel fuel, water, rust, or debris.
NOTE: If the misfire occurred shortly after refueling, the likely cause is poor fuel quality. The MIL DIST (MIL distance) value in the DATA LIST, along with the fuel level, can help you make this determination. Incorrect or contaminated fuel typically affects multiple cylinders. But a problem such as water in the fuel can cause a single cylinder misfire that occurs randomly.
Inspect for evidence of fuel contamination such as diesel fuel, water, rust, or debris.
NOTE: If the misfire occurred shortly after refueling, the likely cause is poor fuel quality. The MIL DIST (MIL distance) value in the DATA LIST, along with the fuel level, can help you make this determination. Incorrect or contaminated fuel typically affects multiple cylinders. But a problem such as water in the fuel can cause a single cylinder misfire that occurs randomly.
NOTE: ECT Sensor 1 must read less than 100 °F (38 °C) before adjusting the valve clearance. Shortcutting this step may cause incorrect valve clearance and engine misfire.
?2.
Recheck the MAP SENSOR fluctuation at idle.
Does the MAP SENSOR continuously fluctuate 0.04 V or more at idle, no loads, engine hot?
YES
Adjusting the valve clearance did not correct the excessive MAP sensor fluctuation. Go to step 66.
NO
The excessive MAP sensor fluctuation was caused by incorrect valve clearance.?
Record your test results in the chart shown below.
Leakage (%)
Leakage Area
No. 1
No. 2
No. 3
No. 4
Does any cylinder have leakage greater than 10 %?
YES
If the majority of the air is escaping from exhaust pipe or throttle body, check for tight valve clearance and adjust each cylinder as needed, then retest. Determine the cause of the cylinder leakage and repair the engine as needed.?
NO
The cylinders are sealing properly. Go to step 68.
