Probable Causes
If the problem is intermittent and the problem cannot be duplicated, refer to Troubleshooting, "Power Is Intermittently Low or Power Cutout Is Intermittent".
If the engine speed is unstable and the problem only occurs during idle, there may be a problem with the circuit for the fast idle switch. Refer to Troubleshooting, "Speed Control (Auxiliary) - Test".
- Check for diagnostic codes.
- Accelerator pedal position sensor
- Power train data link
- Fuel quality
- Valve lash
- Problem with a cylinder
Recommended Repairs
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 problem under other conditions can give misleading results.
Check for Diagnostic Codes Related to the Injectors.
If a diagnostic code is present, verify that a stream of oil is striking the injector. Refer to Troubleshooting, "Injector Solenoid - Test".
Check the Accelerator Pedal Position Sensor
If the engine is in the cruise control mode and the problem does not occur, check the accelerator pedal position sensor.
Use the "Cruise/Idle On/Off Switch" to put the engine in idle mode. Vary the engine rpm with the set/resume switches.
If both of the following conditions are met, connect Cat ET and 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, "Sensor Signal (PWM) - Test".
Check the Systems on the Power Train Data Link
A system that is connected to the power train data link may be affecting the engine's power. These systems include the brakes and the transmission. Monitor the appropriate parameters on Cat ET while the vehicle is experiencing problems. Determine if the systems are limiting the engine's speed.
Determine if the condition is normal operation. Refer to the vehicle's Original Equipment Manufacturer (OEM) if the condition is not normal operation.
Check the Fuel Quality
Check the fuel quality. For further information, refer to the engine's Operation and Maintenance Manual.
Cold weather adversely affects the characteristics of the fuel. Refer to the Operation and Maintenance Manual for information on improving the characteristics of the fuel during cold weather operation.
Check the Valve Lash
Refer to Systems Operation/Testing and Adjusting, "Engine Valve Lash - Inspect/Adjust" for the correct procedure.
Check for a Problem With a Cylinder
Incorrect fuel pressure inside the cylinder head can cause a performance problem. This procedure checks the fuel pressure inside the cylinder head.
- If the engine is running, stop the engine.
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| Illustration 1 | g01400209 |
Locations of the ports for measuring the fuel pressure in the cylinder head | |
Perform a PAR dyno test or a vehicle road test.
Use the following procedure if you perform a vehicle road test:
Monitor the fuel position, the rated fuel limit, and the FRC limit. While the vehicle is operating under full load, the parameters should meet the following conditions:
- The fuel position is equal to the rated fuel limit.
- The fuel position is less than the FRC limit.
If the fuel position is equal to the rated fuel limit and the fuel position is less than the FRC limit, the electronic control system is operating correctly.
If the fuel position is not equal to the rated fuel limit and/or the fuel position is not less than the FRC limit, check the intake manifold pressure on Cat ET when the engine is not running. The intake manifold pressure should be 0 kPa (0 psi).