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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 STS 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 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 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 .
    • 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 Solenoid 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 Inspection and Cleaning . Also inspect for throttle body tampering.
    • Test the items that cause an engine to run rich. For a rich condition, refer to DTC P2178 or P2180 and DTC P2188 or P2190 .

Sensor/System
    • Inspect the air intake system and the crankcase for air leaks.
    • Test the crankcase ventilation system for proper operation. Refer to Crankcase Ventilation System Inspection/Diagnosis 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 MPH (Km/h) Indicator Inoperative or 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
    • Inspect for the correct installation of the ignition coil seals. Incorrect installation may allow water to enter the spark plug area. Inspect for moisture and corrosion.
    • 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:
    • If the spark plugs are gas, coolant, or oil fouled, determine the cause before replacing the spark plugs. Refer to the following procedures:
       - Test the items that cause an engine to run rich. For a rich condition, refer to DTC P2178 or P2180 and DTC P2188 or P2190 .
       - 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 Cooling System Draining and Filling 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 the 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):
    • 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):

Additional
    • Inspect the vacuum hoses for splits and kinks. Verify that the connections and routing are as shown on the 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 in Automatic Transmission - 5L40-E/5L50-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.