Probable Causes
The probable root causes are listed in order below:
Note: If the problem is intermittent and the problem can not be duplicated, refer to Troubleshooting, “Intermittent Low Power or Power Cutout”.
• Individual malfunctioning cylinder
• Fuel supply
• Accelerator pedal position sensor
• Power train Data Link
• Engine speed/timing sensors
Recommended Actions
Perform the following diagnostic procedures in order:
Note: If the problem only occurs under certain conditions, test the engine under those conditions.
Examples of certain conditions are high rpm, full load and engine operating temperature.
Troubleshooting the symptoms under other conditions can give misleading results.
1. Check for an individual malfunctioning cylinder.
a. Ensure that the front valve cover connector and the rear valve cover connector is fully connected and ensure that the connectors are free of corrosion.
b. Ensure that the J2/P2 ECM connector is fully connected and free of corrosion.
c. Connect the electronic service tool to the data link connector. Access the “Cylinder Cutout Test” by selecting the “Diagnostics Tests” under the “Diagnostic” menu. Run the “Cylinder Cutout Test” in order to disable each cylinder. Disabling each cylinder allows the technician to isolate the misfiring cylinders.
If the misfiring cylinder can be isolated to a specific cylinder, refer to Troubleshooting, “Injector Solenoid Circuit – Test”.
2. Check the fuel supply.
a. Monitor the exhaust for smoke while the engine is being cranked.
If white smoke is present, there may be a problem with the fuel quality.
b. Check the fuel quality. Refer to Testing and Adjusting, “Fuel Quality – Test”. Also refer to the Truck Performance and Driveability Diagnostic Guide, LEBT3477.
c. Check the fuel pressure. Refer to Testing and Adjusting, “Fuel System Pressure – Test”.
d. Ensure that the fuel system has been primed. Refer to Testing and Adjusting, “Fuel System – Prime”.
e. Check for fuel supply lines that are restricted.
f. Check the fuel filters.
g. If the temperature is below 0 C (32 F), check for solidified fuel (wax).
h. Check for air in the fuel system. Refer to Testing and Adjusting, “Air in Fuel – Test”.
3. If the engine is in the cruise control mode and the problem does not occur, check the accelerator
pedal position sensor.
a. Use the Cruise/Idle On/Off Switch to put the engine in idle mode.
b. Vary the engine rpm with the Set/Resume switches.
If both of the following conditions are met, monitor the status of the accelerator pedal position sensor.
• The engine is stable by using idle mode to control the engine rpm.
• The engine is unstable by using the accelerator pedal position sensor.
If the status of the accelerator pedal position sensor is unstable, refer to Troubleshooting,
“Accelerator Pedal (Throttle) Position Sensor Circuit – Test”.
4. Check the “Powertrain Data Link”. Ensure that the power train data link is not limiting the power.
a. Monitor the “Powertrain Data Link” status screen while the vehicle is experiencing problems.
If the “Powertrain Data Link” status screen indicates that the “Powertrain Data Link” is limiting the power or the rpm, verify that this is not normal operation. If this is not normal operation, refer to the vehicle OEM for repairs.
5. Check the primary engine speed/timing sensor.
If the signal from the primary engine speed/timing sensor is lost intermittently, the engine may misfire. The misfiring may occur when the signal source becomes the secondary engine speed/timing sensor. Ensure that the primary engine speed/timing sensor is properly installed.
Also, ensure that the wiring is not damaged or pulled too tightly. This may cause an intermittent connection. Also, ensure that the crankshaft gear and the timing plate are not damaged. Refer to Troubleshooting, “Engine Speed/Timing Sensor Circuit – Test” for more information.
6. Check the secondary engine speed/timing sensor.
If the signal for the secondary engine speed/timing sensor is lost during engine start-up, additional time may be required to start the engine. The engine may also misfire during this time until the ECM determines the proper firing order. Refer to Troubleshooting, “Engine Speed/Timing Circuit – Test” for more information.
