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

1. Test for the following conditions:
   • The fuel system for the following:
     • Correct fuel pressure-Refer to Fuel System Diagnosis 
     • Fuel injectors that are leaking or improper operation-Refer to Fuel Injector Diagnosis (J39021 or Tech 2)  or Fuel Injector Diagnosis (CH47976) .
     • Contaminated or a poor fuel quality condition-Refer to Alcohol/Contaminants-in-Fuel Diagnosis (Without Special Tool)  or Alcohol/Contaminants-in-Fuel Diagnosis (Special Tool) .
   • The ignition system for the following:
     • Spark plugs for incorrect heat range or an abnormal condition-Refer to Spark Plug Inspection .
     • For diagnosis of coolant or oil fouled spark plugs, refer to Loss of Coolant or Symptoms - Engine Mechanical .
     • 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 .
   • The operation of the transmission torque converter clutch (TCC)-The scan tool should indicate an engine speed drop when the TCC is commanded ON.
   • The operation of the A/C compressor
   • Items that can cause an engine to run rich or lean.
   • Inspect the 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.
   • 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
   • The engine for the following mechanical failures-Refer to Symptoms - Engine Mechanical :
     • Excessive oil in the combustion chamber or leaking valve seals
     • 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
   • Vacuum hoses for splits or kinks-Verify that the routing and connections are as shown on the Vehicle Emission Control Information label.
   • Knock sensor (KS) system for excessive spark retard activity
   • 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 .
   • 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-Refer to Cooling System Draining and Filling (Vac-N-Fill) or Cooling System Draining and Filling (Static Fill) .
2. If the above conditions do not address the symptom, refer to the additional symptoms tests.