GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 SLS 🢒 Engine 🢒 Engine Controls and Fuel - 2.8L, 3.0L, 3.2L, or 3.6L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspections

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
    • Refer to Important Preliminary Inspections Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Inspect the engine control system grounds for being clean, tight, and in the correct locations. Refer to Engine Controls Schematics . Ensure the resistance between the engine control module (ECM) housing and the battery negative cable is less than 0.5 ohms.
    • Verify that the ECM and ECM bracket fasteners are at the correct torque.
    • Remove the air filter element and inspect for restrictions. Refer to Air Cleaner Element Replacement .
    • Inspect the intake air duct and air cleaner outlet resonator for the following conditions:
       - Restrictions
       - Improperly installed hoses
       - Leaks
       - Collapsed hoses

Fuel System
    • Test for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Test for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Verify that each 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 the items that cause an engine to run rich. For a rich condition, refer to DTC P2178 or P2180 and DTC P2188 or P2190 .
    • Test the items that can cause an engine to run lean. For a lean condition, refer to DTC P2177 or P2179 and DTC P2187 or P2189 .

Sensor/System
    • Test the throttle actuator control (TAC) system. Refer to DTC P0506, P0507, or P050A .
    • Test the crankcase ventilation system. Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • Monitor the knock sensor (KS) system for excessive spark retard activity with a scan tool. Refer to Knock Sensor (KS) System Description , DTC P0327 or P0332 , and DTC P0328 or P0333 .

Ignition System
    • 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.
    • Test for the correct ignition voltage output with a J 26792 Spark Tester. If you suspect an ignition system condition, refer to Electronic Ignition (EI) System Diagnosis .
    • Remove the spark plugs and inspect for the following conditions:
       - Incorrect heat range
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to the following procedures:
    • If the spark plugs are gas, coolant, or oil fouled, determine the cause before replacing the spark plugs. Refer to the following procedures:
       - Test the items that cause an engine to run rich. For a rich condition, refer to DTC P2178 or P2180 and DTC P2188 or P2190 .
       - For diagnosis of coolant contaminated spark plugs, refer to Loss of Coolant .
       - For diagnosis of oil fouled spark plugs, refer to Oil Consumption Diagnosis .
    • Inspect the spark plug boots and terminals for burning or micro-arcing, corrosion, and damage to the insulation.
    • Inspect the crankshaft position (CKP) sensor for the correct resistance. The sensor resistance may be out of range after a hot soak. Measure that the resistance remains within 700-1,200 ohms at all temperatures.

Engine Mechanical
    • Inspect for a condition with the camshaft actuator system. A camshaft actuator that is stuck in the full advance or retard position may cause this symptom. Refer to DTC P0011, P0014, P0021, or P0024 .
    • Inspect for the following engine mechanical conditions:
       - Excessive oil in combustion chamber or leaking valve seals
       - Incorrect cylinder compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Camshaft 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:
    • Inspect the following components for incorrect basic engine parts:
       - Camshafts
       - Cylinder heads
       - Pistons, connecting rods, or bearings.
       - Refer to the following procedures:

Additional
    • Inspect the vacuum hoses for splits and kinks. Verify that the connections and routing are as shown on the Vehicle Emission Control Information Label. Refer to Emission Hose Routing Diagram . Inspect for vacuum leaks in the following areas:
       - The upper and lower intake manifolds
       - The throttle body
       - The positive crankcase ventilation (PCV) system
       - The vacuum brake booster
    • Inspect the evaporative emission (EVAP) canister purge solenoid for the following:
       - A stuck open condition
       - Charcoal contamination due to a defective EVAP canister
       - For more information, refer to Evaporative Emission Control System Description .
    • Inspect the following exhaust system components:
       - The exhaust system components for damage
       - The mufflers for heat distress or possible internal failure
       - The three-way catalytic converters for restrictions
       - For more information, refer to Symptoms - Engine Exhaust .
    • Electromagnetic interference (EMI) on the CKP sensor signal circuits can cause an engine 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 for high voltage components near the ignition control circuits if a condition exists. Also, inspect the engine ground shield of the CKP sensor circuits.
    • Inspect for a condition with the engine mounts. Refer to Engine Mount Replacement - Left Side or Engine Mount Replacement - Right Side .