Symptoms Testing

Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Engine Control Module (ECM) Commanded Reduced Engine Power, Fuel Odor, 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 the conditions listed below:
  • incorrect system operation or fuel pressure-Refer to Fuel System Diagnosis .
  • Leaking or incorrectly operating fuel injectors-Refer to Fuel Injector Solenoid Coil Test .
  • Contaminated or poor quality fuel-Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
• Test/inspect the ignition system for the conditions listed below:
  • 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  HEI Spark Tester -Refer to Electronic Ignition System Diagnosis .
• Test/inspect for the conditions listed below:
  • Non-metered air flow into the air induction system. If a leak is found, repair as necessary. The Throttle or 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 correct direction. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement .
  • 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.
  • 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.
• Test/inspect the engine cooling system for the conditions listed below:
  • 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 additional information, refer to Symptoms - Engine Cooling .

• Test/inspect the exhaust system for the conditions listed below:
  • Physical damage or possible internal failure
  • Restricted three-way catalytic converters

    For additional information, refer to Symptoms - Engine Exhaust .

• Test/inspect the engine for the mechanical conditions 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.