Poor Fuel Economy

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Poor Fuel Economy

Inspections	Action
DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.
Preliminary	Refer to Important Preliminary Inspections Before Starting in Symptoms - Engine Controls . Search for bulletins. Inspect the engine control module (ECM) grounds for being clean, tight, and in the proper locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics . Verify the owner's driving habits, by asking the following questions: Is the A/C or the Defroster mode ON full time? Are the tires at the correct pressure? Is there excessively heavy loads being carried? Is the acceleration rate too much, too often? Is the vehicle used for towing? Remove the air filter element and inspect for dirt or for being restricted. Refer to Air Cleaner Element Replacement .
Fuel System	Determine the type, quality, and alcohol content of the fuel. Oxygenated fuels have lower energy and may deliver reduced fuel economy. Refer to Alcohol/Contaminants-in-Fuel Diagnosis (w/o Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (w/ Special Tool) . Test the fuel injectors. Refer to Fuel Injector Coil Test . Inspect the fuel system. Refer to Fuel System Diagnosis . Test for incorrect fuel pressure. Test for a restricted fuel filter. Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis (w/o Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (w/ Special Tool) . Confirm that each injector harness is connected to the correct injector/cylinder. Relocate the injector harnesses as necessary. Inspect for foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft. Refer to Throttle Body Service . Also inspect for throttle body tampering. Inspect for items which can cause an engine to run rich, long term fuel trim near -13 percent. For a rich condition, refer to DTC P0172 or P0175 .
Sensor/System	Inspect the air intake system and the crankcase for air leaks. Test the crankcase ventilation system for proper operation. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical. Test for proper calibration of the speedometer. Incorrect tire size or axle ratios can affect speedometer calibration and vehicle mileage. Monitor the knock sensor (KS) system for excessive spark retard activity with a scan tool. Refer to Scan Tool Data List .
Ignition System	Test for proper ignition system operation. Refer to Electronic Ignition (EI) System Diagnosis . Test for secondary ignition system leakage. Refer to Electronic Ignition (EI) System Diagnosis . Remove the spark plugs and inspect for the following conditions: Correct heat range Wet plugs Cracks Wear Improper gap Burned electrodes Heavy deposits Determine the cause of the fouling before replacing the spark plugs if spark plugs are gas, coolant or oil fouled. Refer to Spark Plug Inspection . Once you determine the cause of the fouling, refer to one of the following procedures: For diagnosis of a rich condition, refer to DTC P0172 or P0175 . For diagnosis of coolant contaminated spark plugs, refer to Loss of Coolant (M 35) or Loss of Coolant (M 82) in Engine Cooling. For diagnosis of oil fouled spark plugs, refer to Oil Consumption Diagnosis in Engine Mechanical.
Engine Cooling System	Inspect the engine coolant level for being low. Refer to Draining and Filling Cooling System (LA3, LY9) , Draining and Filling Cooling System (LY7) or Draining and Filling Cooling System (LS6) in Engine Cooling. Test the engine thermostat for proper operation and correct heat range. Refer to Thermostat Diagnosis (LA3, LY9) , Thermostat Diagnosis (LY7) or Thermostat Diagnosis (LS6) in Engine Cooling.
Engine Mechanical	Inspect the following: Low compression-Refer to Engine Compression Test in Engine Mechanical. Sticking or leaking valves Worn camshaft lobes-Refer to Camshaft and Bearings Cleaning and Inspection in Engine Mechanical Camshaft timing-Refer to Timing Belt Timing Procedure in Engine Mechanical Broken valve springs Excessive oil in combustion chamber-Leaking valve seals-Refer to Oil Consumption Diagnosis in Engine Mechanical Inspect the following components for incorrect basic engine parts: Camshafts-Refer to Crankshaft and Bearings Cleaning and Inspection in Engine Mechanical. Cylinder heads-Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical. Pistons, connecting rods or bearings-Refer to Piston, Connecting Rod, and Bearings Cleaning and Inspection in Engine Mechanical.
Additional	Visually/physically inspect the vacuum hoses for splits, kinks, and proper connections and routing as shown on vehicle emission control information label. Refer to Emission Hose Routing Diagram . Inspect for air leaks in the following areas: The throttle body mounting area-Refer to Throttle Body Assembly Replacement . The mass air flow (MAF) sensor-Refer to Mass Air Flow (MAF)/Intake Air Temperature (IAT) Sensor Replacement . The duct between the MAF and the throttle body-Refer to Mass Air Flow (MAF)/Intake Air Temperature (IAT) Sensor Replacement . Test the transmission torque converter clutch (TCC) for proper operation. The scan tool should indicate an RPM drop, when the system commands the TCC ON. Refer to Torque Converter Diagnosis Procedure in Automatic Transmission. Inspect the exhaust system components for the following possible restrictions: The exhaust system for physical damage-Refer to Restricted Exhaust in Engine Exhaust. The catalytic converters for possible plugged pipes by comparing the exhaust system back pressure on each side of engine-Refer to Restricted Exhaust in Engine Exhaust. Electromagnetic interference (EMI) on the CKP reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine speed. A sudden increase in engine speed with little change in actual engine speed, indicates that EMI is present. Inspect for high voltage components near ignition control circuits. Inspect the intake manifold and the exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection and Exhaust Manifold Cleaning and Inspection in Engine Mechanical. Inspect the brake system, including the parking brake, for dragging or improper operation. Refer to Symptoms - Hydraulic Brakes in Hydraulic Brakes. Ensure that the vehicle operator does not drive with a foot on the brake pedal. Refer to Symptoms - Park Brake in Park Brake.