Symptoms Testing

Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Engine Control Module (ECM) Commanded Reduced Engine Power, Fuel Odor, Hard Start, Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy, Rough, Unstable, or Incorrect Idle and Stalling, or Surges/Chuggles

Test/inspect the fuel system for the following:
- Incorrect system operation or fuel pressure - Refer to Fuel System Diagnosis .
- Leaking or incorrectly operating fuel injectors - Refer to Fuel Injector Diagnosis .
- Contaminated or poor quality fuel - Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Test/inspect the ignition system for the following:
- Spark plugs with incorrect heat range or an abnormal condition - Refer to Spark Plug Inspection , and Ignition System Specifications .
- Coolant or oil fouled spark plugs. For diagnosis, refer to Coolant in Combustion Chamber , and Oil Consumption Diagnosis .
- Secondary ignition system susceptible to moisture. To help locate damaged or deteriorated components, wet down the secondary ignition system with water from a spray bottle and 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 .
Test/inspect for the following conditions:
- Incorrectly operating 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 .
- Slow responding heated oxygen sensors (HO2S). The HO2S should respond quickly to different throttle positions.
- Water intrusion in the HO2S connector
- Incorrectly installed MAF sensor. A MAF sensor that is improperly installed may cause a hard start. Install the MAF in the proper direction. Refer to Mass Airflow Sensor Replacement .
- Incorrect MAF sensor connections.
- A MAF sensor contaminated by an excessively oiled aftermarket air filter. Some aftermarket air filters require oil to be added to the air filter element. If the MAF sensor becomes contaminated as the result of an excessively oiled aftermarket air filter, a variety of driveability concerns and/or erratic transmission shifting may occur.
- Engine oil contaminated by fuel
- 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 - Observe the Knock Retard parameter on the scan tool for activity more than 0°.
- Electromagnetic interference (EMI) on the reference circuit, which may 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.
- Incorrectly operating crankcase ventilation valve - Refer to Crankcase Ventilation System Inspection/Diagnosis .
- A stuck open evaporative emission (EVAP) canister purge solenoid.
- A crankshaft position sensor with an intermittent condition - Observe the scan tool Crankshaft Position 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.
Test/inspect the engine cooling system for the following conditions:
- A thermostat with incorrect heat range. Refer to Thermostat Diagnosis .
- Incorrect engine coolant level - If the level is low, refer to Cooling System Draining and Filling .
  For more information, refer to Symptoms - Engine Cooling .
Test/inspect the exhaust system components for the following:
- Physical damage or possible internal failure
- Restricted three-way catalytic converters
  For more information, refer to Symptoms - Engine Exhaust .
Test/inspect for the following engine mechanical conditions:
- 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, if necessary. Follow the instructions on the can.
- Incorrect engine parts
  For more information, refer to Symptoms - Engine Mechanical .
If the above conditions do not address the symptom, refer to the additional symptoms tests.