GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 DeVille 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspection/Test

Action

DEFINITION: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Preliminary
    • Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.

Sensor/System
    • Test the positive crankcase ventilation valve (PCV) system for proper operation. Refer to Crankcase Ventilation System Inspection/Diagnosis
    • Test the throttle body, and throttle position (TP) sensor for proper operation and damage.
    • Inspect the knock system (KS) for excessive spark retard.
    • Test the exhaust gas recirculation (EGR) system for proper operation. Refer to Exhaust Gas Recirculation (EGR) System Description
    • Inspect the idle air control (IAC) valve and valve housing for deposits and/or damage. Refer to Idle Air Control (IAC) System Diagnosis
    • Inspect for dirty or damaged air cleaner element.
    • Inspect for collapsed or damage intake air ducting.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Diagnosis .
    • Test the fuel injectors and related wiring. Refer to the following:
    • Inspect the minimum idle speed.
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Inspect the following that may cause the engine to run rich:

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

    • Any 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
    • A leaking fuel pressure regulator
    • Inaccurate mass air flow (MAF) sensor
    • Blockage on the inlet screen of the MAF sensor
    • Vacuum hoses for splits, kinks and proper connections
    • The air intake duct for being collapsed or restricted
    • The air filter for being dirty or restricted--Refer to Air Cleaner Element Replacement

Inspect the following that may cause the engine to run lean:

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

    • Any water intrusion into the HO2S connector
    • An exhaust leak between the HO2S and the engine--Refer to Exhaust Leakage in Engine Exhaust.
    • Any vacuum leaks
    • Incorrect fuel pressure--Refer to Fuel System Diagnosis .
    • An inaccurate MAF sensor
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Vacuum hoses for splits, kinks and proper connections

Ignition System
    • Inspect the ignition control (IC) circuits for proper 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 for misaligned, or damaged spark plug boots.
    • Inspect the spark plugs. Refer to Spark Plug Inspection .

Engine Mechanical
    • Inspect for 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.