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 .
  • The fuel injectors for proper operation-Refer to Fuel Injector Diagnosis (W/J39021 or W/Tech 2)  or Fuel Injector Diagnosis (W/CH47976) .
  • The fuel quality condition-Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
• Test ignition system for the following:
  • Proper ignition system operation-Refer to Electronic Ignition System Diagnosis .
  • Spark plugs-Refer to Spark Plug Inspection .
  • Spark plugs for correct application-Refer to Ignition System Specifications .
• Inspect for the following conditions
  • 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
  • Items that can cause an engine to run lean or rich. Refer to DTC P0171 or P0172 .
  • The crankshaft position sensor for proper operation
  • The proper operation of the knock sensor system
  • Inspect the exhaust system. 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 system for proper operation
  • The proper operation of the evaporative emission (EVAP) canister purge solenoid
  • The proper operation of the engine cooling system-Refer to Symptoms - Engine Cooling .
• 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.