The fuel mileage measured in an actual road test and in multiple fuel measurements is significantly lower than the predicted value. Also, it is significantly lower than the fuel mileage obtained in the previous road test.
Remove the air cleaner and inspect for contamination, collapsing in the air duct, or any condition that restricts the flow.
Verify the driver's driving habits.
Check whether the A/C is always ON or the defroster mode is ON.
Verify that the tire air pressure is appropriate.
Check if the tire size has been changed.
Check if excessively heavy loads are carried.
Check if acceleration is excessive or too frequent.
Inspect for clutch slip.
Inspect for brake drag.
Inspect the drive belt tension.
Inspect whether the transmission shift pattern and the down shift operation are normal.
Inspect the fuel quality.
Inspect the engine oil level and oil quality.
Instruct the owner to check fuel consumption again after refilling fuel.
Inspect whether the odometer is operating properly.
Check the scan tool data list.
Check for related Service Bulletins.
Check if DPD regeneration is frequent. Frequent DPD regeneration is a possible cause.
Compare the Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), and Fuel Temperature parameters while the engine is cold. If the difference between the temperature values is 5°C {9°F} or more, inspect for high resistance in each circuit and for sensor failure when the engine is cold.
{When the ignition switch is turned ON, the MAF sensor is heated, and as a result, the IAT shows a higher value than the normal intake air temperature.}
Inspect the fuel type and quality.
Inspect for fuel leakage.
Inspect the engine coolant level.
Inspect whether the engine thermostat is always open, and inspect for an incorrect heat range.
Inspect whether the engine cooling fan is always ON.
Inspect the air cleaner, air intake duct, and intercooler for clogging, holes, or leakage.
Inspect for any condition that restricts flow within the turbocharger intake duct.
Inspect the intake throttle valve for sticking.
Inspect the inlet manifold for clogging or air leakage.
Inspect the MAF sensor for dirt or damage.
Perform the Variable Swirl Valve with a scan tool. Inspect the diaphragm valve operation when the diaphragm valve is commanded ON/OFF.
Inspect the turbocharger turbine wheel, shaft, or compressor wheel for wear or damage.
Inspect the VGS actuator operation.
Inspect whether the exhaust system flow is restricted.
Inspect the compression pressure of the engine.
Inspect the valve clearance.
Inspect the valve spring for breakage and fatigue.
Inspect the camshaft for wear.