GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Electrical 🢒 Poor Fuel Economy

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DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, 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 PCM grounds for being clean, tight, and in the proper locations. Refer to 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?
       - Is there excessively heavy loads being carried?
       - Is the acceleration rate too much, too often?
       - Remove the air filter element and check for dirt or for being restricted. 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 Fuel System Specifications and Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect that each injector harness is connected to the correct injector and cylinder according to the firing order firing order: 1-8-7-2-6-5-4-3. 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 items that cause an engine to run rich long term fuel trim near -13 percent. For a rich condition, refer to DTC P0172 or P0175 .

Sensor/System
    • Inspect the air intake system and crankcase for air leaks.
    • Inspect the crankcase ventilation valve for proper operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.
    • Inspect for proper calibration of the speedometer. Connect J 33431-B Signal Generator and Instrument Panel Tester to the vehicle speed sensor (VSS) electrical connector. Turn ON the ignition with the tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph).
    • Monitor the knock sensor (KS) system for excessive spark retard activity with a scan tool. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Test for proper ignition voltage output with 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. Gap the spark plugs using a wire gauge gap tool. Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs if the spark plugs are gas, coolant, or oil fouled. Refer to one of the following procedures:
       - For diagnosis of the rich condition, refer to DTC P0172 or P0175 .
       - For diagnosis of coolant or oil fouled 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
    • Wetting down 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 Engine Controls Schematics .

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 the engine for the following mechanical conditions:
       - 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 in Engine Mechanical 8.1L .
       - Low cylinder compression--Refer to Engine Compression Test in Engine Mechanical 8.1L.
    • Inspect the following components for incorrect basic engine parts:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to the appropriate procedures in Engine Mechanical 8.1L for diagnosis procedures.

Additional Checks
    • Visually and physically inspect 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 Powertrain On Board Diagnostic (OBD) System Check - Automatic Transmission in Automatic Transmission-4L80-E or Diagnostic System Check - Automatic Transmission in Automatic Transmission-Allison.
    • Inspect the following components of the exhaust system for possible restrictions:
       - The exhaust system for damaged or collapsed pipes
       - The exhaust manifold for a collapsed inner wall
       - The mufflers for heat distress or possible internal failure
       - The three-way catalytic converters for possible plugged conditions--Compare the exhaust system back pressure on each side of engine. Test back pressure by removing secondary air injection (AIR) test valves near exhaust manifolds. 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. Check for high voltage components, near ignition control circuits, if a problem exists.
    • Inspect the park neutral position (PNP) switch circuit.
    • Inspect the intake and the exhaust manifold passages for casting flash.
    • Inspect the brake system for dragging or improper operation. Refer to Brake Caliper Inspection in Disc Brakes. Ensure that the vehicle operator does not drive with a foot on the brake pedal.