Symptom
Occasionally, it may be noticed that more fuel is required to perform the same task as before.
Cause
Operator effect
• Inaccurate record keeping or tank filling
• Winter blend or No. 1 fuel
Application effect
• Heavy loading Gross Vehicle Weight (GVW)
• Low rear axle ratio
• Large frontal area
• Accessory usage (Power Takeoff, etc.)
• Additional equipment drawing fuel from vehicle fuel tanks
• Extended idle applications
• Tire size, tire condition, air pressure
Chassis effect
• Brake drag
• Cooling fan clutch locked ON
• Transmission slippage/shifting
• Fuel tank plumbing or venting
• Intake or exhaust restriction
Engine effect
• Incorrect or failed thermostat
• Failed Variable Geometry Turbocharger (VGT) operation
• Oil aeration
• Fuel system leaks
• Base engine performance loss
Procedure
1. Review operator records and fueling procedures. Measurement errors are common. Fuel consumption taken only from one tank of use is susceptible to significant error because of filling procedures and vehicle application differences during operation. Accurate fuel consumption must be measured over time with a record of what the vehicle was doing during the measurement period.
2. Loss of fuel economy is normal if winter blend fuel or No. 1 diesel fuel is being used.
3. Review vehicle specifications to determine if fuel consumption is normal for type of application and use of vehicle. (Compare consumption with similar vehicles in the same application and Truck Computer Analysis of Performance and Economy (TCAPE) report.
4. Do all tests on Performance Diagnostic form or in Section 6 of this manual. These tests will verify the operating condition of the following engine and chassis systems:
• Intake system
• Exhaust system
• Fuel delivery and filtration
• High-pressure oil system
• Injector operation
• VGT operation
• Oil aeration
• Base engine condition
• Electronic control system condition
If all tests are passed, the engine is operating normally.
