Poor Fuel Economy

Checks	Action
DEFINITION: Fuel economy, as measured by an actual road test is noticeably lower than expected. Also, economy is noticeably lower than the fuel economy was on this vehicle at one time, as previously shown by an actual road test.
Preliminary	Perform the Diagnostic System Check - Vehicle in Vehicle DTC Information. Search for bulletins. Inspect the powertrain control module (PCM) grounds for being clean, tight, and in the proper locations. Refer to Power and Grounding Component Views in Wiring Systems. Review the owner's driving habits. Is A/C ON full time (Defroster mode ON)? Are tires at correct pressure? Are tires and wheels the correct size? Are excessively heavy loads being carried? Is acceleration too much, too often?
Fuel System	Test the fuel pressure. Refer to Fuel System Diagnosis . Inspect the fuel injectors using the following tests: Refer to Fuel Injector Coil Test . Refer to Fuel Injector Balance Test with Tech 2 . Ensure each injector harness is connected to the correct injector/cylinder. Inspect for fuel in the pressure regulator vacuum hose.
Sensor/System	Inspect the air intake system and crankcase for air leaks. Test for proper calibration of speedometer. Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity.
Ignition System	Test for proper ignition voltage output using the following steps: Clip the J 26792 Spark Tester to engine ground. Connect one end of a spark plug wire to the J 26792 . Connect the other end of the spark plug wire to the coil being tested. Connect another spark plug wire between the remaining tower of the coil being tested and ground. Crank the engine while observing the J 26792 . A spark should be observed. Repeat the above steps for each coil. If spark is not present at the coils, test for the following conditions: Coils-Cracks, carbon tracking/arcing, or a secondary resistance value outside the specified range Coil Resistance: 5,000-8,000 ohms (5K-8K ohms) IMPORTANT: Spraying the secondary ignition wires with a light mist of water may help locate an intermittent condition. Ignition components will arc to ground when a secondary component is faulty. Spark plug wires-Signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range Spark Plug Wire Resistance: 9840 ohms per meter (3000 ohms per foot) Spark Plug Wire Resistance Police: 1,968 ohms per meter (600 ohms per foot) Defective module Ignition system wiring-Loose ignition module feed or ground connection, or damaged system wiring Remove spark plugs and inspect for the following: Fouled plugs Cracks Wear Improper gap Burned or damaged electrodes Improper heat range or reach If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to Spark Plug Inspection .
Engine Cooling System	Inspect the engine coolant level for being low. Refer to Loss of Coolant in Engine Cooling. Inspect and test for an incorrect or faulty engine thermostat. Refer to Thermostat Diagnosis in Engine Cooling.
Engine Mechanical	Inspect engine mechanical for the following: Test compression-Refer to Engine Compression Test in Engine Mechanical. Sticking or leaking valves Worn camshaft lobes Valve timing Bent push rods-Refer to Valve Rocker Arm and Push Rods Cleaning and Inspection in Engine Mechanical - 3.8L. Worn rocker arms-Refer to Valve Rocker Arm and Push Rods Cleaning and Inspection in Engine Mechanical - 3.8L. Broken valve springs-Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical - 3.8L. Excess oil in combustion chamber-Inspect for leaking valve seals. Inspect for incorrect engine parts, such as the following basic components: Camshaft Cylinder heads Pistons, etc. Refer to Symptoms - Engine Mechanical in Engine Mechanical - 3.8L.
Additional Inspections	Inspect the transmission torque converter clutch (TCC) operation. The scan tool should indicate a RPM drop, when the system commands the TCC ON. Test the exhaust system for a possible restriction. Inspect for the following: Inspect the exhaust system for damaged or collapsed pipes. Inspect the mufflers for heat distress or possible internal failure. For possible plugged 3-way catalytic converter-Refer to Restricted Exhaust in Engine Exhaust.