GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Generic 🢒 2005 🢒 Unit Repair 🢒 Engine 🢒 Engine Controls - 3.5L (LX9) 🢒 Poor Fuel Economy

Inspections

Action

DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, fuel economy is noticeably lower than the economy was on this vehicle at one time, as previously shown by an actual road test.

Preliminary
    • Refer to Important Preliminary Inspections Before Beginning in Symptoms - Engine Controls .
    • 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 and Engine Controls Schematics .
    • Discuss driving habits with the owner.
       - Is the A/C ON or the Defroster mode ON full time?
       - Are the tires at the correct pressure?
       - Are the wheels and tires the correct size?
       - Are there excessively heavy loads being carried?
       - Is the acceleration rate too much, too often?
       - Remove the air filter element and inspect for dirt or for restrictions.

Fuel System
    • Discuss with the owner 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 .
    • Test for restricted fuel or incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect that each fuel injector harness is connected to the correct injector and cylinder. Relocate the injector harnesses as necessary.
    • Inspect for the following conditions that may cause the engine to run rich:
       - Water intrusion in the heated oxygen sensor (HO2S) connector
       - Engine oil contaminated by fuel
       - An evaporative emission (EVAP) canister purge condition
       - Leaking fuel injectors--Refer to Fuel System Diagnosis .
       - For proper operation of the manifold absolute pressure (MAP) sensor--Refer to Manifold Absolute Pressure Sensor Diagnosis .
       - For proper operation of the mass air flow (MAF) sensor--Refer to Scan Tool Data Definitions and Scan Tool Data List .
       - Vacuum hoses that are split, kinked, or improperly connected
       - An air intake duct that is collapsed or restricted
       - An air filter that is dirty or restricted

Sensor/System
    • Inspect the air intake system and crankcase for air leaks.
    • Inspect the crankcase ventilation valve for proper operation.
    • Inspect for an inaccurate speedometer.
    • Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity.

Ignition System
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester.
    • Remove the spark plugs and inspect for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to Spark Plug Inspection .
    • An improper spark plug gap will cause a driveability problem. Gap the spark plugs using a wire gage gap tool Refer to Spark Plug Replacement and Ignition System Specifications .
    • Determine the cause of the fouling before replacing the spark plugs.
    • Visually and physically inspect the secondary ignition for the following conditions:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • Soaking the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply water.
    • Inspect for loose ignition coil grounds. Refer to Power and Grounding Component Views in Wiring Systems.

Engine Cooling System
    • Inspect the engine coolant level for being low. Refer to Loss of Coolant in Engine Cooling.
    • Inspect the engine thermostat for proper operation and for the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect engine mechanical for the following conditions:
       - Compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber, leaking valve seals
       - For more information refer to one of the following procedures in Engine Mechanical:
    • For incorrect basic engine parts inspect for the following components:
       - The camshaft--Refer to Camshaft and Bearings Cleaning and Inspection in Engine Mechanical.
       - The cylinder heads--Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical.
       - The pistons, etc.--Refer to Piston, Connecting Rod, and Bearing Cleaning and Inspection in Engine Mechanical.

Additional Inspections
    • Visually and physically check the vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
    • Inspect the transmission torque converter clutch (TCC) operation. The scan tool should indicate a RPM drop, when the system commands the TCC ON.
    • Inspect the exhaust system for a possible restriction. Inspect for the following conditions:
       - The exhaust system for damaged or collapsed pipes
       - The mufflers for heat distress or possible internal failure
       - For possible plugged catalytic converters--Refer to Restricted Exhaust in Engine Exhaust.
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change indicates EMI is present. Inspect for high voltage components, near ignition control circuits, if a condition exists.
    • Inspect the brake system for dragging or improper operation. Refer to Symptoms - Hydraulic Brakes in Hydraulic Brakes. Verify that the vehicle operator does not drive with a foot on the brake pedal.