CM850 Electronic Control System – CM850 Electronic Control System – Troubleshooting and Repair Manual

Fuel Economy Troubleshooting Tree

Symptoms

    • Fuel Consumption Excessive

How To Use This Tree

This symptom tree is to be used to troubleshoot fuel economy complaints. This tree is used along with the Fuel Consumption – Customer Complaint Form and the Driveability Low Power/Excessive Fuel – Consumption Checklist to help isolate engine, chassis, or driver issues associated with excessive fuel consumption.

Shop Talk

The Fuel Consumption – General Information section of this manual and Troubleshooting Excessive Fuel Consumption, Bulletin 3666094, must be referenced prior to any troubleshooting being performed on a customer’s engine.

The cause of excessive fuel consumption is difficult to diagnose and correct because of the potential number of factors involved. Actual fuel consumption problems can be caused by any of the following factors:

  • Engine factors
  • Vehicle factors and specifications
  • Environmental factors
  • Driver technique and operating practices
  • Fuel system factors
  • Low power or driveability problems.

Before troubleshooting, it is important to determine the exact complaint. Is the complaint based on whether the problem is real or perceived, or does
not meet driver expectations? The Fuel Consumption – Customer Complaint Form is a valuable list of questions that can be used to assist the service technician in determining the cause of the problem. Complete the form before troubleshooting the complaint. The following are some of the factors that
must be considered when troubleshooting fuel consumption complaints.

  • Excessive idling time: Idling the engine can use from 0.5 to 1.5 gallons per hour depending on the engine idle speed.
  • Vehicle Aerodynamics: The largest single power requirement for a truck is the power needed to overcome air resistance. As a general rule, each 10 percent reduction in air resistance results in a 5 percent increase in mile per gallon.
  • Rolling Resistance: Rolling resistance is the second largest consumer of power on a truck. The type of tire and tread design have a sizeable effect on fuel economy and performance. Changing from a bias ply to a low profile radial tire can reduce rolling resistance by about 36 percent.
  • Environmental and Seasonal Weather Changes: There can be as much as 1 to 1.5 mile per gallon difference in fuel consumption, depending on the season and the weather conditions.
  • Truck Route and Terrain: East and west routes experience almost continual crosswinds and head winds. Less fuel can be used on north and south routes where parts of the trip are
    not only warmer, but have less wind resistance.
  • Driver Technique and Operating Practices: A 1 mile per hour increase in road speed equals a 0.1 mile per gallon increase in fuel consumption. This means that increasing road speed from 50 to 60 mph will result in a loss of fuel mileage of 1 mpg.
  • Result of a Low Power or Driveability Problem: An operator will change driving style to compensate for a low power or driveability problem. Some things the driver is likely to do are (a) shift to a high engine rpm or (b) run on the droop curve in a lower gear instead of upshifting to drive at part-throttle conditions. These changes in driving style will increase the amount of fuel used.

Additional vehicle factors, vehicle specifications and axle alignment, can also affect fuel consumption.
For additional information on troubleshooting fuel consumption complaints, refer to Troubleshooting Excessive Fuel Consumption, Bulletin 3666094.

Troubleshooting Steps

STEPS SPECIFICATIONS
STEP 1. Verify the complaint.  
  STEP 1A. Fill out the Fuel Consumption – Customer Complaint Form.
  STEP 1B. Run VE/VMS.
STEP 2. Electronic Checks, use INSITE™ electronic service tool.  
  STEP 2A. Check for fault codes.
  STEP 2B. Confirm Features and Parameters.
  STEP 2C. Check the ECM calibrations.
  STEP 2D. Monitor vehicle speed.
  STEP 2D-1. Inspect the engine and chassis grounds.
STEP 3. Engine performance.  
  STEP 3A. Engine performance troubleshooting.


Guided Step 1 – Verify the complaint.

Guided Step 1A – Fill the Fuel Consumption – Customer Complaint Form.

Action

Fill out the Fuel Consumption – Customer Complaint Form in the Troubleshooting and Repair Manual, ISC, QSC8.3, ISL and QSL9 Engines, Bulletin 4021418.

Is the problem caused by vehicle factors, environmental factors, or driver technique?

YES NO
No Repair No Repair
Repair complete

Guided Step 1B – Run VE/VMS.

Action

Fill out the Driveability/Low Power/Excessive Fuel Consumption – Checklist in Troubleshooting and Repair Manual, ISC, QSC8.3, ISL and QSL9 Engines, Bulletin 4021418.

Use the data gathered to run VE/VMS or provide the data to the appropriate support personnel to run VE/VMS.

NOTE: VE/VMS does
not account for adverse ambient conditions.

Taking into consideration the adverse effect of ambient conditions, does the VE/VMS fuel economy conflict with the customer’s current fuel economy?

YES NO
No Repair

Explain to the customer the effects of driving habits on fuel consumption.

Repair complete


Guided Step 2 – Electronic Checks, use INSITE™ electronic service tool.

Guided Step 2A – Check for fault codes.

Conditions

  • Turn keyswitch ON.
  • Connect INSITE™ electronic service tool.

Action

Use INSITE™ electronic service tool to read the fault codes.

Are there any active or high counts of inactive fault codes?

YES NO
No Repair No Repair
Troubleshoot fault codes

Guided Step 2B – Confirm Features and Parameters.

Conditions

  • Turn keyswitch ON.
  • Connect INSITE™ electronic service tool.

Action

Confirm the Programmable Features and Parameters are set correctly.

Are the Features and Parameters set correctly?

YES NO
No Repair

Reset the Features and Parameters to their appropriate values.

Repair complete

Guided Step 2C – Check the ECM calibration.

Conditions

  • Turn keyswitch ON.
  • Connect INSITE™ electronic service tool.

Action

Compare the ECM code with the engine rating and CPL. Verify the calibration is correct.

Is the calibration correct?

YES NO
No Repair

Recalibrate the ECM with the correct ECM code.

Repair complete

Guided Step 2D – Monitor vehicle speed.

Conditions

  • Turn keyswitch ON.
  • Connect INSITE™ electronic service tool.

Action

Use INSITE™ electronic service tool to monitor Vehicle Speed while the vehicle is not moving.

Does vehicle speed read 0 when the vehicle is not moving?

YES NO
No Repair No Repair

Guided Step 2D-1 – Inspect the engine and chassis grounds.

Conditions

  • Turn keyswitch OFF.

Action

Check for loose or corroded engine, chassis, or battery ground connection.

  • Check the engine ground connection
  • Check the chassis ground connections
  • Check the battery terminal connections.

Are all grounds present, properly grounded, free of corrosion, and tight?

YES NO

Check the VSS and the VSS circuit.

Refer to Procedure 019-091 (Vehicle Speed Sensor, Magnetic Pick Up) in Section 19.

Replace, clean, or tighten the grounds.

Repair complete

Repair complete


Guided Step 3 – Engine performance.

Guided Step 3A – Engine performance troubleshooting.

Action

Perform the Fuel System Checks, Air Handling Checks, and Base Engine Checks in the Engine Performance Troubleshooting Tree.

Poor fuel economy complaint still exists?

YES NO

Perform the Fuel System Checks, Air Handling Checks, and Base Engine Checks in the Engine Performance Troubleshooting Tree.

No Repair
Repair complete

Repair complete

Last Modified:  07-Dec-2007