Symptoms Testing

Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Engine Control Module (ECM) Commanded Reduced Engine Power, Hard Start, Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy, Rough, Unstable, or Incorrect Idle and Stalling, or Surges/Chuggles

• Test the fuel system for the following:
  • Incorrect system operation or fuel pressure-Refer to Fuel System Diagnosis (LGX) Fuel System Diagnosis (LF4) .
  • Leaking or incorrectly operating fuel injectors-Refer to Fuel Injector Solenoid Coil Test .
  • Contaminated or poor fuel quality-Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
• Test the ignition system for the following:
  • Spark plugs with incorrect heat range or an abnormal condition-Refer to Spark Plug Inspection , and Ignition System Specifications .
  • Coolant or oil fouled spark plugs-Refer to Coolant in Combustion Chamber , or Oil Consumption Diagnosis for diagnosis.
  • Secondary ignition system susceptible to moisture. Wet down the secondary ignition system with water from a spray bottle to help locate damaged or deteriorated components. Engine running, look/listen for arcing or misfiring as the water is applied.
  • Weak spark using the J 26792  spark tester -Refer to Electronic Ignition System Diagnosis .
• Inspect for the following conditions:
  • Non-metered air flow into the air induction system. If a leak is found, repair as necessary. The Q38 Throttle Body: Throttle/Idle Learn procedure must be performed when the repair is complete.
  • Incorrectly operating transmission torque converter clutch (TCC) - The scan tool should indicate an engine speed drop when the TCC is commanded ON-Refer to Torque Converter Diagnosis
  • Incorrectly operating A/C compressor
  • An engine that runs lean or rich-Observe the Fuel Trim parameters with a scan tool.
  • Slow responding heated oxygen sensors (HO2S)-The HO2S should respond quickly to different throttle positions.
  CAUTION:

  Refer to Heated Oxygen and Oxygen Sensor Caution .

  • Water intrusion in the HO2S connector
  NOTE:

  The embossed arrows on the mass air flow (MAF) sensor indicate the direction of the intake air flow. The arrows must point toward the engine.

  • Incorrect mass air flow (MAF) sensor installation. A MAF sensor that is incorrectly installed may cause a hard start. Install the MAF in the proper direction. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement (Front LF4)Mass Airflow Sensor with Intake Air Temperature Sensor Replacement (Rear LF4)Mass Airflow Sensor with Intake Air Temperature Sensor Replacement (LGX) .
  • Incorrect MAF sensor connections
  • Engine oil contaminated by fuel
  • Split or kinked vacuum hoses-Verify that the routing and connections are correct.
  • Excessive knock sensor (KS) system spark retard activity-Observe the scan tool Knock Retard parameter for activity greater than 0°.
  • Electromagnetic interference (EMI) on reference circuits, which can cause a 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 the high voltage components near the ignition control circuit if a condition exists.
  • An incorrectly operating crankcase ventilation valve-Refer to Crankcase Ventilation System Inspection/Diagnosis .
  • A stuck open evaporative emission (EVAP) canister purge solenoid valve.
  • A crankshaft position (CKP) sensor with an intermittent condition-Observe the scan tool Crankshaft Position Resync Counter parameter. The parameter should remain at 0 during all operating conditions, and when moving the related harnesses and connectors between the crankshaft position sensor and the ECM.
• Inspect the engine cooling system for the following conditions:
  • A thermostat with an incorrect heat range-Refer to Thermostat Diagnosis .
  • Incorrect engine coolant level-If the level is low, refer to Loss of Coolant .

    For more information, refer to Symptoms - Engine Cooling .

• Inspect the exhaust system components for the following conditions:
  • Physical damage or possible internal failure
  • Restricted three-way catalytic converters

    For more information, refer to Symptoms - Engine Exhaust .

• Inspect the engine for the following mechanical conditions:
  • Excessive oil in the combustion chamber or leaking valve seals
  • Oil consumption
  • Incorrect cylinder compression
  • Sticking or leaking valves
  • Worn camshaft lobes
  • Incorrect valve timing
  • Worn rocker arms
  • Broken valve springs
  • Excessive carbon buildup in the combustion chambers-Clean the chambers with Top Engine Cleaner, if necessary. Follow the instructions on the can.
  • Incorrect engine parts

    Refer to Symptoms - Engine Mechanical for additional information.

• If the above conditions do not address the symptom, refer to the additional symptoms tests.