Probable Causes
The probable root causes are listed in order below:
• Vehicle Operation
• Vehicle Specifications
• Fuel Supply
• Air Inlet System or Exhaust System
Recommended Actions
Perform the following diagnostic procedures in order:
1. Inspect the ECM Totals.
a. Inspect the ECM Totals for idle time.
b. Inspect the Fleet Trip Histograms for excessive idling or poor driving habits.
c. Check the following items:
• Load factor
• Average vehicle speed
• Vehicle Speed Limit
Look for excessively high values for these items.
d. Consider the following environmental conditions:
• Wind
• Snow
2. Check the vehicle specifications.
Check the specifications on the following items:
• Gearing or Power train
• Vehicle Aerodynamics
• Heavy Loads
3. Check the fuel supply.
a. Monitor the exhaust for smoke while the engine is being cranked.
If no smoke is present, there may be a problem with the fuel quality or there may be a problem with the fuel supply.
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”.
4. Check the Air Inlet System for restrictions or leakage. Check the Exhaust System for
restrictions or leakage. Refer to Testing and Adjusting, “Air Inlet and Exhaust System – Inspect”.
