Poor Fuel Economy

Checks	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 Checks Before Starting in Symptoms - Engine Controls . Search for bulletins. Verify that the engine control module (ECM) grounds are clean, tight, and in the correct locations. Inspect the following owner's driving habits. Is the A/C or the Defroster mode ON all the time? Are the tires at the correct pressure? Is there excessively heavy loads being carried? Is the acceleration rate too much, too often? Remove the air filter element and inspect for restrictions. Refer to Air Cleaner Element Replacement and replace if necessary.
Fuel System	Inspect 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 (WO/Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (W/Special Tool) . Test the fuel injectors. Refer to Fuel Injector Coil Test . Test for incorrect fuel pressure. Refer to Fuel System Diagnosis . Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis (WO/Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (W/Special Tool) . Verify that the fuel injector harness is connected to the correct injector/cylinder according to the firing order. Inspect for foreign material accumulation in the throttle bore, and for carbon deposits on the throttle plate and shaft. Also inspect for throttle body tampering. Test or inspect items that can cause an engine to run rich. 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 valve. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve. Use a scan tool in order to monitor the KS system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description and DTC P0329 or DTC P0334 .
Ignition System	Test the ignition voltage output with the J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis . Remove the spark plugs and inspect for the following conditions: Incorrect heat range Wet plugs Cracks Excessive wear Improper gap Burned electrodes Heavy deposits Refer to the following procedures: Spark Plug Inspection Spark Plug Replacement Ignition System Specifications An improper spark plug gap can cause a driveability concern. Refer to Ignition System Specifications . If the spark plugs are fouled by gas, coolant, or oil, determine the cause before replacing the plugs. Refer to the following procedures. For diagnosis of a rich condition, refer to DTC P0172 or P0175 . For diagnosis of coolant or oil fouled spark plugs, refer to Loss of Coolant in Engine Cooling or Symptoms - Engine Mechanical in Engine Mechanical - 2.5L (LBK). Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied. Inspect for the correct installation of the ignition coil seals. Incorrect installation may allow water to enter the spark plug area. Inspect for moisture and corrosion. Inspect the ignition coils and the spark plug boots for cracks or carbon tracking.
Engine Cooling System	Inspect the engine coolant level. Refer to Draining and Filling Cooling System (2.0L) or Draining and Filling Cooling System (2.5L) in Engine Cooling. Test the engine thermostat. Verify that the thermostat is of the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.
Engine Mechanical	Inspect for the following engine mechanical conditions: Excessive oil in the combustion chamber and for leaking valve seals Incorrect cylinder compression Sticking or leaking valves Worn camshaft lobes Incorrect valve timing Broken valve springs Excessive carbon buildup in the combustion chambers. Clean the chambers with top engine cleaner. Follow the instructions on the can. For more information, refer to the following procedures in Engine Mechanical - 2.5L (LBK): Oil Leak Diagnosis Engine Compression Test Symptoms - Engine Mechanical in Engine Mechanical - 2.5L (LBK). Inspect the following for incorrect basic engine parts: The camshaft The cylinder heads The pistons, connecting rods, and bearings
Additional Checks	Inspect the vacuum hoses for splits or kinks. Verify that the connections and routing are as shown on the Vehicle Emission Control Information label. Refer to Emission Hose Routing Diagram . Test the transmission torque converter clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON. Refer to Diagnostic System Check - Automatic Transmission in Automatic Transmission - ZF 4HP 16. Inspect the following exhaust system components: The exhaust system for physical damage The mufflers for heat distress or possible internal failure The three-way catalytic converter(s) for restrictions. For more information, refer to Restricted Exhaust in Engine Exhaust. Electromagnetic interference (EMI) on the reference circuit can cause a misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed. A sudden increase of the engine speed parameter with little change in actual engine speed indicates that EMI is present. Inspect the high voltage components near the ignition control circuit if a problem exists. Inspect the brake system for dragging or improper operation. Ensure that the vehicle operator does not drive with a foot on the brake pedal.