Poor Fuel Economy - Isuzu N 2008 Series Workshop Manual

Poor Fuel Economy

Checks
Definition:
Fuel economy, as measured by actual road tests and several tanks of fuel, is noticeably lower than expected. Also, the
economy is noticeably lower than it was on this vehicle at one time, as previously shown by actual road tests.
Preliminary Checks
Sensor Checks
Fuel System Checks
Cooling System Checks
• Remove the air cleaner and check for dirt, or for air ducts being plugged or
restricted. Replace as necessary.
• Inspect the driving habits of the owner.
• Is the A/C ON full time, defroster mode ON?
• Are the tires at the correct pressure?
• Are the tire sizes changed?
• Are excessively heavy loads being carried?
• Is the acceleration too much, too often?
• Inspect for clutch slip.
• Inspect brake drag.
• Inspect dive belt tension.
• Inspect for a proper transmission shift pattern and down shift operation (Smoother
only).
• Inspect the fuel quality (cetane index).
• Inspect the engine oil level and quality.
• Suggest to the owner to fill the fuel tank and recheck the fuel economy.
• Inspect the odometer is correctly operated.
• Inspect the Scan Tool Data List in this section.
• Inspect the Service Bulletin.
Inspect the engine control sensors for the following conditions. Refer to Scan Tool Data
List in this section.
• Compare the Coolant Temperature with the Intake Air Temperature (IAT) and Fuel
Temperature (FT) parameters on a cold engine condition. If the difference among
temperature reading is more than 5°C (9°F) on a cold engine, check for high
resistance in each circuit or for a skewed sensor.
Notice: The mass air flow (MAF) sensor is heated and as a result the IAT may indicate
a higher than normal intake air temperature if the ignition switch is being ON.
Inspect the fuel system for the following conditions. Refer to Fuel System in Section 1D
Engine Fuel.
• Fuel type and quality.
• Check fuel leak.
Inspect the cooling system for the following conditions. Refer to Cooling System
Section.
• Engine coolant level.
• Engine thermostat for always being open or for the wrong heat range.
• Engine cooling fan for always being ON.
Engine Control System (4JJ1)
Action
1A-339