GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Epica 🢒 Engine 🢒 Engine Controls - 2.5L (LBK) 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

DEFINITION: The engine runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle may vary in speed. Either condition may be severe enough to stall the engine.

Preliminary Checks
    • Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Verify that the engine control module (ECM) grounds are clean, tight, and in the correct locations.
    • Remove and inspect the air filter element for restrictions. Refer to Air Cleaner Element Replacement and replace if necessary.

Fuel System
    • Test for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Verify that the fuel injector harness is connected to the correct injector/cylinder according to the firing order.
    • Test the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Test or inspect items that can cause an engine to run lean. For a lean condition, refer to DTC P0171 or P0174 .
    • Test or inspect items that can cause an engine to run rich. For a rich condition, refer to DTC P0172 or P0175 .

Sensor/System
    • Test the crankcase ventilation valve. Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description and DTC P0329 or DTC P0334 .
    • Test the evaporative emission (EVAP) canister purge valve for proper operation. Refer to Evaporative Emission Control System Description .

Ignition System
    • Test the ignition voltage output with the J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis .
    • Remove the spark plugs and inspect for the following conditions:
       - Incorrect heat range
       - Wet plugs
       - Cracks
       - Excessive wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to the following procedures:
    • An improper spark plug gap can cause a driveability concern. Refer to Ignition System Specifications .
    • If the spark plugs are fouled by gas, coolant, or oil, determine the cause before replacement. Refer to the following:
       - For diagnosis of a rich condition, refer to DTC P0172 or P0175 .
       - For diagnosis of coolant or oil fouled spark plugs, refer to Loss of Coolant in Engine Cooling or Symptoms - Engine Mechanical in Engine Mechanical - 2.5L (LBK).
    • 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.
    • Test the crankshaft position (CKP) sensor for the correct resistance. The CKP sensor resistance may be out of range after a hot soak. The resistance should be between 460-620 ohms.
    • Inspect for the correct installation of the ignition coil seals. Incorrect installation may allow water to enter the spark plug area. Inspect for moisture and corrosion.
    • Inspect the ignition coils and the spark plug boots for cracks or carbon tracking.

Engine Mechanical
    • Inspect for the following engine mechanical conditions:
       - 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. Follow the instructions on the can.
       - For more information, refer to the following procedures in Engine Mechanical - 2.5L (LBK):
   •  Oil Leak Diagnosis
    • Inspect the following for incorrect basic engine parts:
       - The camshaft
       - The cylinder heads
       - The pistons, connecting rods, and bearings

Additional Checks
    • Inspect the evaporative emission (EVAP) canister purge solenoid for the following:
      • A stuck open condition
      • Charcoal contamination due to a defective EVAP canister
    • Inspect the following exhaust system components:
       - The exhaust system for physical damage
       - The mufflers for heat distress or possible internal failure
       - The three-way catalytic converter(s) for restrictions.
       - For more information, refer to Restricted Exhaust in 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 may indicate that EMI is present. If a problem exists, inspect the high voltage components near the ignition control circuit.
    • Inspect the engine mounts. Refer to Engine Mount Replacement in Engine Mechanical - 2.5L (LBK).