GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra Classic - 2WD 🢒 Engine 🢒 Engine Controls and Fuel - 4.8L, 5.3L, and 6.0L 🢒 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 Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Verify that the powertrain control module (PCM) grounds are clean, tight, and in the proper locations. Refer to Power and Grounding Component Views and Engine Controls Schematics .
    • Inspect the owners driving habits.
       - 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. Refer to Air Cleaner Element Replacement . Replace as 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 .
    • Inspect for 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 foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft.
    • Inspect for the following that may cause the engine to run rich:

Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.

       - Water intrusion in the heated oxygen sensor (HO2S) connector
       - Engine oil contaminated by fuel
       - An evaporative emission (EVAP) canister purge condition
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Leaking fuel injectors--Refer to Fuel System Diagnosis .
       - An inaccurate mass air flow (MAF) sensor
       - Blockage on the inlet screen of the MAF sensor--Refer to Mass Airflow Sensor/Intake Air Temperature Sensor Replacement .
       - Vacuum hoses that are split, kinked, or improperly connected
       - An air intake duct that is collapsed or restricted--Refer to Air Cleaner Resonator Outlet Duct Replacement .
       - An air filter that is dirty or restricted--Refer to Air Cleaner Element Replacement .

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

Ignition System
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester .
    • Remove the spark plugs and inspect for the following:
       - 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. Refer to Spark Plug Inspection . Gap the spark plugs using a wire gage gap tool. Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs. Refer to Spark Plug Inspection .
    • Visually and physically inspect the secondary ignition for the following:
       - 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 Electronic Ignition (EI) System Diagnosis .

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

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Compression--Refer to Engine Compression Test .
       - 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. Refer to Oil Consumption Diagnosis .
    • For incorrect basic engine parts inspect for the following:
       - The camshaft--Refer to Camshaft and Bearings Cleaning and Inspection .
       - The cylinder heads--Refer to Cylinder Head Cleaning and Inspection .
       - The pistons, etc.--Refer to Piston, Connecting Rod, and Bearing Cleaning and Inspection .

Refer to Symptoms - Engine Mechanical for diagnostic procedures.

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. Refer to Torque Converter Diagnosis for the 4L60-E transmission or Torque Converter Diagnosis for the 4L80-E transmission.
    • Inspect 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.
       - Inspect for possible plugged catalytic converters. Refer to Restricted 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 park neutral position (PNP) switch circuit. Refer to Torque Converter Diagnosis for the 4L60-E transmission or Park/Neutral Position Switch Adjustment for the 4L80-E transmission.
    • Inspect the intake and the exhaust manifold passages for casting flash.
    • Inspect the brake system for dragging or improper operation. Refer to Brakes Drag . Verify that the vehicle operator does not drive with a foot on the brake pedal.