GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 GMC Sierra Denali 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspections

Action

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

Preliminary Inspections
    • Refer to Important Preliminary Inspections Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Verify that the powertrain control module (PCM) grounds are clean, tight, and in the proper locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics .
    • Remove and inspect the air filter element for dirt or for restrictions. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Inspect the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect that each fuel injector harness is connected to the correct injector/cylinder. Relocate fuel injector harnesses as necessary.
    • Inspect for the following that may cause the engine to run rich:

Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.

       - Water intrusion in the heated oxygen sensor (HO2S) connector
       - Engine oil contaminated by fuel
       - An evaporative emission (EVAP) canister purge condition
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Leaking fuel injectors--Refer to Fuel System Diagnosis .
       - An inaccurate mass air flow (MAF) sensor
       - Blockage on the inlet screen of the MAF sensor--Refer to Manifold Absolute Pressure Sensor Replacement .
       - Vacuum hoses that are split, kinked, or improperly connected
       - An air intake duct that is collapsed or restricted--Refer Intake Air Resonator Replacement .
       - An air filter that is dirty or restricted--Refer to Air Cleaner Element Replacement .
    • Inspect for the following conditions that may cause the engine to run lean:

Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.

       - Water intrusion in the HO2S connector
       - An exhaust leak between the HO2S and the engine--Refer to Exhaust Leakage in Engine Exhaust.
       - Vacuum leaks
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Restricted fuel injectors--Refer to Fuel System Diagnosis .
       - An inaccurate MAF sensor
       - Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected

Sensor/System
    • Inspect the crankcase ventilation valve for proper operation. Place a finger over the inlet hole of the valve end several times. The valve should snap back. If not, replace the valve. Refer to Crankcase Ventilation System Description in Engine Mechanical.
    •  Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity.

Ignition System
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester . Refer to Electronic Ignition (EI) System Diagnosis for procedure.
    • Remove spark plugs and check for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to Spark Plug Inspection .
    • An improper spark plug gap will cause a driveability problem. Refer to Spark Plug Inspection . Gap the spark plugs using a wire gage gap tool. Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs.
    • Visually and physically inspect secondary ignition for the following conditions:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • Soak the secondary ignition system with water from a spray bottle. Soaking the secondary ignition system may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply water.
    • Monitor the Misfire Current Counters while driving the vehicle in the conditions that the misfire occurred. If a misfiring cylinder can be located, use the DTC P0300 table for diagnosis. Refer to DTC P0300 .
    • Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Mechanical
    • Inspect engine mechanical for the following conditions:
       - Compression--Refer to Engine Compression Test in Engine Mechanical.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber or leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • For incorrect basic engine parts. Inspect the following:
       - The camshaft--Refer to Camshaft and Bearings Cleaning and Inspection in Engine Mechanical.
       - The cylinder heads--Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical.
       - The pistons, etc.--Refer to Piston, Connecting Rod, and Bearing Cleaning and Inspection in Engine Mechanical.

Refer to Symptoms - Engine Mechanical in Engine Mechanical for diagnosis procedures.

Additional Inspections
    • Inspect the exhaust system for possible restrictions. Inspect for the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged three-way catalytic converters by comparing exhaust system back pressure on each side of engine. Inspect the back pressure by removing secondary air injection (AIR) check valves near exhaust manifolds. Refer to Restricted Exhaust in Engine Exhaust.
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change indicates that EMI is present. If a problem exists, inspect routing of secondary ignition wires or high voltage components near the ignition control circuits.
    • Inspect the park neutral position (PNP) switch circuit. Refer to Park/Neutral Position Switch Adjustment in Automatic Transmission 4L60-E/4L65-E.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.
    • Inspect for an exhaust gas recirculation (EGR) valve stuck open or EGR pintle not seating properly. Inspect EGR valve gasket for leaks.