Symptoms Testing

Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, 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:
  • The fuel system for correct operation volume and pressure-Refer to Fuel System Diagnosis .
  • The fuel injectors for proper operation-Refer to Fuel Injector Solenoid Coil Test .
  • The fuel quality condition-Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
• Test the ignition system for the following:
  • Spark plugs for incorrect heat range or an abnormal condition-Refer to Spark Plug Inspection and Ignition System Specifications .
  • For diagnosis of coolant or oil fouled spark plugs-Refer to Coolant in Combustion Chamber or Oil Consumption Diagnosis .
  • Wet down the secondary ignition system with water from a spray bottle-Wetting down the secondary ignition system may 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 .
• Inspect for the following conditions:
  • On vehicles equipped with automatic transmission, the proper operation of the transmission torque converter clutch (TCC) - The scan tool should indicate an engine speed drop when the TCC is commanded ON.
  • The proper operation of the 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.
  • 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.
  • Inspect the 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 .
  • Inspect the mass air flow (MAF) sensor connections.
  • Engine oil contaminated by fuel
  • Vacuum hoses for splits or kinks-Verify that the routing and connections are as shown on the Vehicle Emission Control Information label.
  • Knock sensor system for excessive spark retard activity-Observe the Knock Retard parameter on the scan tool for activity more than 0°.
• The exhaust system components for the following:
  • Physical damage or possible internal failure
  • The three-way catalytic converters for a restriction

    For more information, refer to Symptoms - Engine Exhaust .

• Electromagnetic interference (EMI) on the reference circuit 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.
• The crankcase ventilation valve for proper operation-Refer to Crankcase Ventilation System Inspection/Diagnosis (With Turbocharger) .
• The proper operation of the evaporative emission (EVAP) canister purge system-Refer to Evaporative Emission Control System Description .
• The engine cooling system for the following conditions:
  • The thermostat is the correct heat range. Refer to Thermostat Diagnosis .
  • Proper engine coolant level-If the level is low, refer to Loss of Coolant .
• The crankshaft position sensor for an intermittent condition-Observe the scan tool CKP 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.
• Inspect the engine for the following mechanical failures:

  Refer to Symptoms - Engine Mechanical .

  • 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. Follow the instructions on the can.
  • Incorrect engine parts
• If the above conditions do not address the symptom, refer to the additional symptoms tests below.