GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 XLR 🢒 Engine 🢒 Engine Controls - 4.6L (LH2) 🢒 Detonation/Spark Knock

Detonation/Spark Knock

Inspection/Tests

Action

DEFINITION: A mild to severe ping, usually worse under acceleration. The engine makes sharp metallic knocks that changes with throttle opening.

Preliminary Inspections
    • Refer to 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 and Engine Controls Schematics .
    • If there are no engine mechanical faults, fill the fuel tank with a known high quality fuel that meets the vehicles minimum octane requirements. Road test the vehicle and re-evaluate the vehicles performance.

Fuel System
    • Test for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Test the fuel injectors. Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Solenoid Coil Test .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • 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 heated oxygen sensor (HO2S) connector
       - An exhaust leak between the HO2S and the engine--Refer to Exhaust Leakage in Engine Exhaust.
       - Vacuum leaks
       - Restricted fuel injectors--Refer to Fuel Injector Balance Test with Special Tool .
       - 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 .
       - Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected

Sensor/System

Inspect for an engine coolant temperature (ECT) sensor that has shifted in value. Refer to Temperature Versus Resistance .

Ignition System
    • 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 water is applied.
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis .
    • Remove the spark plugs and inspect for the following conditions:
       - Correct heat range
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to Spark Plug Inspection .
       - Verify that the spark plugs are of the type. Refer to Ignition System Specifications .
    • An improper spark plug gap will cause a driveability problem. Gap the spark plugs using a wire gage gap tool. Refer to Ignition System Specifications and Spark Plug Replacement .
    • 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.
    • Inspect for loose ignition coil ground.

Engine Cooling System

Inspect for obvious overheating conditions:

    • Low engine coolant--Refer to Loss of Coolant in Engine Cooling .
    • A faulty thermostat--Refer to Thermostat Diagnosis in Engine Cooling.
    • Restricted air flow to the radiator or restricted coolant flow through the radiator
    • An inoperative electric cooling fan--Refer to Cooling Fan Inoperative in Engine Cooling.
    • Proper tension on the drive belts--Refer to Drive Belt Tensioner Diagnosis in Engine Mechanical.
    • The correct coolant solution should be a 50/50 mix. Refer to Cooling System Description and Operation in Engine Cooling.

Engine Mechanical

Inspect for the following engine mechanical conditions:

    • Excessive oil in combustion chamber--Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • High cylinder compression--Refer to Engine Compression Test in Engine Mechanical.
    • Combustion chambers for excessive carbon buildup--Clean the combustion chamber by using top engine cleaner. Follow the instructions on the can.
    • Incorrect camshaft timing--Refer to Setting Camshaft Timing in Engine Mechanical.
    • Inspect 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.
    • Refer to Symptoms - Engine Mechanical in Engine Mechanical.

Additional Inspections
    • Inspect the park/neutral position (PNP) switch operation.
    • Inspect the transmission torque converter clutch (TCC) operation. The TCC applying too soon can cause the engine to spark knock.