Poor Fuel Economy

WARNING: This page is about a different car, the 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

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 correct locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics . Verify that the ECM and ECM bracket fasteners are at the correct torque. 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 restrictions. Refer to Air Cleaner Element Replacement . Inspect the intake air duct and air cleaner outlet resonator for the following conditions: Restrictions Improperly installed hoses Leaks Collapsed hoses
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 for incorrect fuel pressure. Refer to Fuel System Diagnosis . Test for a restricted fuel filter. Refer to Fuel System Diagnosis . Verify that each injector harness is connected to the correct injector/cylinder according to the firing order. Test the fuel injectors. Refer to Fuel Injector Coil Test . 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. Test or inspect for items that cause an engine to run rich. For a rich condition, refer to DTC P2178 or DTC P2188 .
Sensor/System	Inspect the air intake system and the crankcase for air leaks. Test the crankcase ventilation system for proper operation. Refer to in Engine Mechanical - 3.6L (LY7). Test for correct calibration of the speedometer. Incorrect tire size or axle ratios can affect speedometer calibration and vehicle mileage. Refer to Symptoms - Instrument Panel, Gages and Console in Instrument Panel, Gages, and Console. Monitor the knock sensor (KS) system for excessive spark retard activity with a scan tool. Refer to Knock Sensor (KS) System Description , DTC P0327 or P0332 , and DTC P0328 or P0333 .
Ignition System	Test for the correct ignition voltage output with a J 26792 Spark Tester. If you suspect an ignition system condition, refer to Electronic Ignition (EI) System Diagnosis . Remove the spark plugs and inspect for the following conditions: Incorrect heat range Wet plugs Cracks Wear Improper gap Burned electrodes Heavy deposits Refer to the following procedures: Spark Plug Replacement Ignition System Specifications Spark Plug Inspection If the spark plugs are gas, coolant or oil fouled, determine the cause before replacing the spark plugs. Refer to the following procedures: For diagnosis of a rich condition, refer to DTC P2178 or DTC P2188 . For diagnosis of coolant contaminated spark plugs, refer to Loss of Coolant in Engine Cooling. For diagnosis of oil fouled spark plugs, refer to Oil Consumption Diagnosis in Engine Mechanical - 3.6L (LY7). Inspect the spark plug boots and terminals for burning or micro-arcing, corrosion, and damage to the insulation.
Engine Cooling System	Inspect the engine coolant level for being low. Refer to Draining and Filling Cooling System in Engine Cooling. Test the engine thermostat. Verify the correct operation and heat range. Refer to Thermostat Diagnosis in Engine Cooling.
Engine Mechanical	Inspect for the following engine mechanical conditions: Excessive oil in combustion chamber or leaking valve seals Incorrect cylinder compression Sticking or leaking valves Worn camshaft lobes Camshaft 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 - 3.6L (LY7): Oil Consumption Diagnosis Engine Compression Test Camshaft Timing Drive Chain Alignment Diagram Setting Camshaft Timing Symptoms - Engine Mechanical Inspect the following components for incorrect basic engine parts: Camshafts Cylinder heads Pistons, connecting rods, or bearings Refer to the following procedures in Engine Mechanical - 3.6L (LY7): Camshaft Timing Drive Components Cleaning and Inspection Camshaft Cleaning and Inspection Cylinder Head Cleaning and Inspection Piston, Connecting Rod, and Bearings Cleaning and Inspection
Additional	Inspect the vacuum hoses for splits and kinks. Verify that the connections and routing are as shown on Vehicle Emission Control Information Label. Refer to Emission Hose Routing Diagram . Test the transmission torque converter clutch (TCC). The scan tool should indicate a drop in engine speed, when the system commands the TCC ON. Refer to Torque Converter Diagnosis Procedure in Automatic Transaxle - 4T65-E. Inspect the following exhaust system components: The exhaust system for damage The mufflers for heat distress or possible internal failure The three-way catalytic converters for restrictions For more information, refer to Symptoms - Engine Exhaust in Engine Exhaust. Electromagnetic interference (EMI) on the crankshaft position (CKP) sensor signal circuits can cause an engine misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed parameter. A sudden increase in the engine speed parameter, with little change in actual engine speed indicates that EMI is present. Inspect for high voltage components near the ignition control circuits if a condition exists. Also, inspect the engine ground shield of the CKP sensor circuits. Inspect the brake system, including the parking brake, for dragging or incorrect operation. Refer to Symptoms - Hydraulic Brakes in Hydraulic Brakes. Ensure that the vehicle operator does not drive with a foot on the brake pedal.