GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 XLR 🢒 Engine 🢒 Engine Controls - 4.6L (LH2) 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspections

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 Inspections
    • 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 proper locations. Refer to Power and Grounding Component Views in Wiring Systems.
    • Remove and inspect the air filter element for dirt or for restrictions. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Test the fuel injectors. Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Solenoid Coil Test .
    • 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/cylinder. Relocate fuel injector harnesses as necessary.
    • Inspect for the following conditions 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 .
       - Inspect for proper operation of the manifold absolute pressure (MAP) sensor. Refer to Scan Tool Data Definitions .
       - Inspect for proper operation of the mass air flow (MAF) sensor. Refer to Scan Tool Data Definitions .
       - Blockage on the inlet screen of the MAF sensor
       - Vacuum hoses that are split, kinked, or improperly connected
       - An air intake duct that is collapsed or restricted
       - An air filter that is dirty or restricted--Refer to Air Cleaner Element Replacement .
    • Inspect for the following conditions that may cause the engine to run lean:

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

       - Water intrusion in the HO2S connector
       - An exhaust leak between the HO2S and the engine--Refer to Exhaust Leakage in Engine Exhaust.
       - Vacuum leaks
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Restricted fuel injectors--Refer to Fuel Injector Balance Test with Special Tool .
       - Inspect for proper operation of the MAP sensor. Refer to Scan Tool Data Definitions .
       - Inspect for proper operation of the MAF sensor. Refer to Scan Tool Data Definitions .
       - Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected

Sensor/System
    •  Inspect the air intake ducts for being collapsed, damaged areas, looseness, improper installation, or leaking especially between the MAF sensor and the throttle body.
    •  Inspect the Transaxle Range Switch input with the vehicle in drive and the gear selector in drive or overdrive.
    •  Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Scan Tool Data Definitions .

Ignition System
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis for the procedure.
    • Remove spark plugs and check for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • In order to properly inspect the spark plugs or to correct a condition, refer to the following procedures:
       -  Spark Plug Inspection
       -  Spark Plug Replacement
    • An improper spark plug gap will cause a driveability problem. Gap the spark plugs using a wire gage gap tool.
    • Determine the cause of the fouling before replacing the spark plugs.
    • Inspect the spark plug boots for signs of arcing, cracks, carbon tracking, or damage.
    • Soak the secondary ignition system with water from a spray bottle. Soaking the secondary ignition system may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply water. Monitor the Misfire Current Counters while driving the vehicle in the conditions that the misfire occurred.
    • Inspect for loose ignition coil grounds. Refer to Power and Grounding Component Views in Engine Electrical.

Engine Mechanical
    • Inspect engine mechanical for the following conditions:
       - Compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber or leaking valve seals
       - For more information, refer to one or more of the following procedures in Engine Mechanical:
    • For incorrect basic engine parts. Inspect the following components:
       - The camshaft--Refer to Camshaft Cleaning and Inspection in Engine Mechanical.
       - The cylinder heads--Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical.
       - The pistons, etc.--Refer to Piston, Connecting Rod, and Bearing Cleaning and Inspection in Engine Mechanical.
    • Inspect for excessive crankshaft endplay that will cause the crankshaft position (CKP) sensor reluctor wheel to move out of alignment with the CKP sensor. Refer to Crankshaft and Bearing Cleaning and Inspection in Engine Mechanical. This could result in any of the following conditions:
       - A no start
       - A start and stall
       - Erratic performance
    • Refer to Symptoms - Engine Mechanical in Engine Mechanical for diagnosis procedures.

Additional Inspections
    • Inspect the exhaust system for the following possible restrictions:
       - The exhaust system for damaged or collapsed pipes
       - The mufflers for heat distress or possible internal failure
       - For possible plugged catalytic converters--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 that EMI is present. If a problem exists, inspect routing of high voltage components near the ignition control circuits.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection , or Exhaust Manifold Cleaning and Inspection - Left Side and Exhaust Manifold Cleaning and Inspection - Right Side in Engine Mechanical.