GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2003 🢒 Envoy - 2WD 🢒 Engine 🢒 Engine Electrical 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspections

Action

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

Preliminary

Refer to Symptoms - Engine Controls .

Sensor
    • Inspect the heated oxygen sensor (HO2S). Check for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery coating. The sensor will result in a high but false signal voltage, or rich exhaust indication. The control module will reduce the amount of fuel delivered to the engine causing a severe driveability condition.
    • Inspect the engine coolant temperature (ECT) sensor using the scan tool in order to compare the engine coolant temperature with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees more than or less than the ambient air temperature on a cold engine, test for a high resistance in the coolant sensor circuit or the sensor itself.
    • Inspect the manifold absolute pressure (MAP) sensor response and accuracy.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect to determine if a rich or lean system causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the fuel trim will help identify the condition.
       - Lean--The long term fuel trim is more than 150.
       - Rich--The long term fuel trim is less than 115.
    • Perform the fuel injector balance test. Refer to Fuel Injector Balance Test with Tech 2 .
    • Test the fuel injector driver circuit.
        1. Disconnect the injector harness connector at the injectors.
        2. Connect an injector test lamp between the terminals of each injector connector and note the lamp while cranking.
        3. If the test lamp fails to blink at any connector, it is a faulty injector drive circuit harness, connector, or terminal.
    • Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test .
    • Inspect the evaporative emission (EVAP) control system. Refer to Evaporative Emission Control System Description .

Ignition System
    • Inspect the overall ignition system
    • Inspect for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Blistered insulators
       - Heavy deposits
       - Refer to Spark Plug Inspection .

Additional
    • Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low idle air control (IAC) counts.
    • Inspect the control module grounds for being clean, tight, and in their proper locations.
    • Observe the scan tool to determine if the control module is receiving an A/C signal. Refer to HVAC Compressor Clutch Does Not Engage or HVAC Compressor Clutch Does Not Disengage in HVAC Systems-Manual. If a condition exists with the A/C ON, inspect the A/C system operation.
    • Inspect the battery cables and ground straps for being clean and secure.
    • Inspect the crankcase ventilation valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve. Refer to Crankcase Ventilation System Inspection/Description in Engine Mechanical.

Engine Mechanical

Inspect for the following:

    • Broken motor mounts--Refer to Engine Mount Inspection in Engine Mechanical.
    • Improper valve timing
    • Low compression--Refer to Engine Compression Test in Engine Mechanical.
    • Worn rocker arms--Refer to Valve Rocker Arm and Valve Lash Adjuster Cleaning and Inspection in Engine Mechanical.
    • Broken or weak valve springs
    • Worn camshaft lobes--Refer to Camshafts Cleaning and Inspection in Engine Mechanical.