Symptoms Testing

Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Engine Control Module (ECM) Commanded Reduced Engine Power, 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:
   • An incorrectly operating fuel system. Test for the following:
     • Correct fuel pressure. Refer to Fuel System Diagnosis .
     • Fuel injectors that are leaking or improper operation. Refer to Fuel Injector Diagnosis .
     • Contaminated or a poor fuel quality condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
   • An incorrectly operating ignition system. Test or inspect for the following:
     • Spark plugs with an incorrect heat range or an abnormal condition. Refer to Spark Plug Replacement .
     • Coolant or oil fouled spark plugs. Refer to Spark Plug Inspection , and Ignition System Specifications
     • Secondary ignition system susceptible to moisture. Engine running, wet down the secondary ignition system with water from a spray bottle to help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied.
     • Weak spark using the EL 26792  HEI Spark Tester. For equivalent regional tools, refer to Special Tools . Refer to Electronic Ignition System Diagnosis .
   • An incorrectly operating A/C compressor
   • Items that can cause an engine to run lean or rich. Refer to DTC P0171 or P2097 (LUW) , and DTC P0172 or P2096 (LUW) .
   • Incorrectly installed Mass Air Flow (MAF) / Intake Air Temperature (IAT) Sensor.
   • Incorrect crankshaft position sensor resistance. The crankshaft position sensor resistance may be out of range after a hot soak. The resistance should be between 700-1 200 Ω.
   • Split or kinked vacuum hoses. Verify that the routing and connections are as shown on the vehicle emission control information label.
   • Excessive knock sensor system spark retard activity. Refer to Knock Sensor Replacement , and DTC P0324, P0326, or P06B6 , and DTC P0325, P0327, or P0328 (LUW)  and.
   • Electromagnetic interference on the reference circuit can cause a misfire condition. You can usually detect electromagnetic interference 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 electromagnetic interference is present. Inspect the high voltage components near the ignition control circuit if a condition exists.
   • A leaking or restricted positive crankcase ventilation system.
   • A stuck open Evaporative Emission (EVAP) Canister Purge Solenoid Valve
   • An incorrectly operating engine cooling system. Test or inspect for the following:
     • A thermostat with an incorrect heat range. Refer to Thermostat Diagnosis (LUJ) , Thermostat Diagnosis (LUW) .
     • Improper engine coolant level. Refer to Cooling System Draining and Filling (GE-47716 Fill) , Cooling System Draining and Filling (Static Fill) .
   • Incorrectly operating or damaged exhaust system components. Test for the following:
     • Physical damage or possible internal failure
     • Restricted three way catalytic converters

       For more information, refer to Symptoms - Engine Exhaust .

   • An engine mechanical failure. Test for the following:
     • Excessive oil in the combustion chamber or leaking valve seals
     • Incorrect cylinder compression
     • Sticking or leaking valves
     • Worn camshaft lobes
     • Incorrect valve timing
     • 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 more information, refer to Symptoms - Engine Mechanical .

2. If the above conditions do not address the symptom, refer to the additional symptoms tests.