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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra Classic - 4WD 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic 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 Beginning in Symptoms - Engine Controls .
    • Search for bulletins.
    • Inspect the powertrain control module (PCM) grounds for being clean, tight, and in the proper locations. Refer to Power and Grounding Component Views .
    • 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 System Diagnosis .
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a restricted fuel filter. 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
       - Purging of a saturated evaporative emission (EVAP) canister
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - A leaking fuel pressure regulator--Refer to Fuel System Diagnosis .
       - Leaking fuel injectors--Refer to Fuel System Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected
       - Proper operation of the manifold absolute pressure (MAP) sensor--Refer to DTC P0106 .
       - An air intake duct that is collapsed or restricted
       - 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 .
       - Vacuum leaks
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Restricted fuel injectors--Refer to Fuel Injector Balance Test with Special Tool and Fuel Injector Balance Test with Tech 2 .
       - Proper operation of the MAP sensor--Refer to DTC P0106
       - Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected

Sensor/System
    •  Test for conditions which cause an incorrect idle speed.
       -  Throttle body tampering, excessive deposits, or damage
       -  Restricted air intake system
       -  Large vacuum leak
    • Inspect the crankcase ventilation system for proper operation. Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • Inspect the IAC System. Refer to Idle Air Control (IAC) System Diagnosis .
    • Inspect the throttle position (TP) sensor for proper operation. Refer to DTC P0121 .
    • Monitor the knock sensor (KS) system for excessive spark retard activity with a scan tool.

Ignition System
    • In order to properly inspect the spark plugs or to correct a condition, refer to the following procedures:
       -  Spark Plug Inspection
       -  Spark Plug Replacement
    • Inspect for proper secondary ignition voltage output with the J 26792 HEI Spark Tester.
    • An improper spark plug gap will cause a driveability condition. Gap the spark plugs using a wire gage gap tool.
    • If the spark plugs are found to be fouled, determine the cause before replacing the spark plugs.
    • Inspect for damaged spark plug wires. Refer to Spark Plug Wire Inspection .
    • Monitor the Misfire Current Counters while driving the vehicle in the conditions that the symptom occurred. If a misfiring cylinder can be located, use the DTC P0300 table for diagnosis. Refer to DTC P0300 .

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber or leaking valve seals.
       -  For more information, refer to one or more of the following procedures in Engine Mechanical:
    • For incorrect, worn, or damaged basic engine parts, inspect the following:
       - The camshaft.
       - The cylinder heads.
       - The pistons, etc.

Additional Inspections
    • Inspect the exhaust system for possible restrictions. Inspect for the following:
       - The exhaust system for damaged or collapsed pipes
       - The mufflers for heat distress or possible internal failure
       - Possible plugged catalytic converters--Refer to Restricted Exhaust .
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine misfire condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM parameter with little change in actual engine RPM change indicates that EMI is present. If a condition exists, inspect routing of secondary ignition wires or high voltage components near the ignition control circuits.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection .
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.