Symptoms Testing

Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, 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/inspect the fuel system for any of the conditions listed below:
  • Incorrect fuel system operation and pressure - Refer to Fuel System Diagnosis .
  • Incorrectly operating fuel injectors - Refer to Fuel Injector Diagnosis .
  • Poor fuel quality condition - Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
• Test/inspect the ignition system for any of the conditions listed below:
  • Spark plugs with incorrect heat range or an abnormal condition - Refer to Gas Engine Ignition Spark Plug Inspection , or Ignition System Specifications .
  • Coolant or oil fouled spark plugs - For diagnosis refer to Coolant in Combustion Chamber , or Oil Consumption Diagnosis .
  • Secondary ignition system susceptible to moisture. Engine running, wet down the secondary ignition system with water from a spray bottle to help locate damaged or deteriorated components. 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 .
• Test/inspect for any of the conditions listed below:
  • Non-metered air flow into the air induction system. If a leak is found, repair as necessary. The Throttle Body Idle Air Flow Compensation Reset procedure must be performed when the repair is complete.
  • An incorrectly operating transmission torque converter clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON.
  • 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 Replacement - Left Side , or Mass Airflow Sensor Replacement - Right Side .
  • Incorrect MAF sensor connections.
  • Engine oil contaminated by fuel
  • Split or kinked vacuum hoses - Verify that the routing and connections are as shown on the Vehicle Emission Control Information label.
  • Excessive knock sensor (KS) system spark retard activity - Observe the Knock Retard parameter on the scan tool for activity more than 0°.
  • Electromagnetic interference (EMI) on the reference circuit, 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
  • A crankshaft position sensor with an intermittent condition - Observe the scan tool Crankshaft Position Resync Counter parameter. The Resync Counter 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.
  • Test/inspect the exhaust system components for any of the conditions listed below:
    • Physical damage or possible internal failure
    • Restricted three-way catalytic converters

      For more information, refer to Symptoms - Engine Exhaust .

  • Test/inspect the engine cooling system for any of the conditions listed below:
    • A thermostat with incorrect heat range.
    • Incorrect engine coolant level - If the level is low, refer to Loss of Coolant (LGY) , or Loss of Coolant (LSY) .
  • Test/inspect the engine for any of the mechanical failures listed below:
    • 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

      For additional information, refer to Symptoms - Engine Mechanical .

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