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 Fuel System Diagnosis (Except Saturn)  or Fuel System Diagnosis (Saturn) .
  • The fuel injectors for proper operation - Refer to Fuel Injector Diagnosis (J39021 or Tech 2)  or Fuel Injector Diagnosis (CH47976)  or Fuel Injector Diagnosis (SA9182E or Tech 2) .
  • 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
  • For 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. Refer to DTC P0171 or P0172 .
  • Inspect the 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 with Intake Air Temperature 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 (Without Turbocharger) .
• Evaporative emission (EVAP) canister purge solenoid that is stuck open
• 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 parameter. The Resync 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 Additional Symptoms Tests .