GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 DeVille 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Poor Fuel Economy

Inspection/Test

Action

DEFINITION: Fuel economy, as measure by an actual road test is noticeably lower than expected. Also, 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 Checks Before Starting in Symptoms - Engine Controls .
    • Search for Bulletins.
    • Discuss driving habits with the owner.
    • Inspect the following:
       - A/C operation
       - Correct tire pressures
       - Correct wheel and tire size
       - Excessive vehicle weight loads
       - Excessive acceleration

Sensor/System
    • Inspect for an engine coolant temperature (ECT) sensor shifted in value. Refer to Temperature Versus Resistance .
    • Inspect the throttle body, throttle position (TP) sensor, and accelerator cables for proper operation.
    • Inspect the knock system (KS) for excessive spark retard.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Diagnosis .
    • Test the fuel injectors. Refer to the following:
    • Inspect for fuel in the pressure regulator.
    • Inspect the minimum idle speed.
    • Inspect for items that cause the engine to run rich.

Ignition System
    • Inspect for proper ignition control (IC) circuit connections at each ignition module assembly.
    • Test for proper ignition voltage output using the J 26792 Spark Tester.
    • Inspect the ignition coils for cracks, or damage.
    • Inspect for a missing or damaged ignition module assembly grounding spring.
    • Inspect the spark plugs. Refer to Spark Plug Inspection

Engine Cooling
    • Inspect for low engine coolant level. Refer to Loss of Coolant in Engine Cooling.
    • Inspect for an incorrect or malfunctioning engine coolant thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect the following:
       - Engine compression--Refer to Engine Compression Test in Engine Mechanical.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Worn rocker arms--Refer to Valve Rocker Arms Cleaning and Inspection in Engine Mechanical.
       - Broken or weak valve springs--Refer to Valve Spring Inspection and Measurement in Engine Mechanical.
       - Excessive oil in the combustion chamber-leaking valve seals
    • Inspect for incorrect basic engine parts. Inspect the following:
       - Camshafts
       - Cylinder heads
       - Pistons, etc.

Additional
    • Inspect the exhaust system for possible restrictions. Refer to Restricted Exhaust in Engine Exhaust.
    • Inspect the torque converter clutch (TCC) for proper operation. Refer to Torque Converter Diagnosis in Automatic Transaxle.