GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Impala 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

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
    • Perform the Diagnostic System Check - Vehicle in Vehicle DTC Information.
    • 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 in Wiring Systems.
    • Remove and inspect the air filter element for dirt or for restrictions.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Diagnosis .
    • Test the fuel injectors using the following tests:
       - Refer to Fuel Injector Solenoid Coil Test .
       - Refer to Fuel Injector Balance Test with Tech 2 .
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect for fuel in the pressure regulator vacuum hose.
    • Ensure each injector harness is connected to the correct injector/cylinder.
    • Test items which can cause an engine to run rich (long term fuel trim is significantly in the negative range). Refer to Diagnostic Aids for DTC P0172 .
    • Inspect items that can cause an engine to run lean (long term fuel trim is significantly in the positive range). Refer to Diagnostic Aids for DTC P0171 .

Sensor/System
    • Inspect for conditions which cause an incorrect idle speed.
       - Throttle body tampering, excessive deposits, or damage
       - Restricted air intake system.
       - Large vacuum leak
    • Test the crankcase ventilation valve for proper operation.
    • Test the throttle position (TP) sensor and related wiring. Refer to DTC P0123 .
    • Test the exhaust gas recirculation (EGR) system for proper operation.
    • Test the transaxle range switch input with the vehicle in drive and the gear selector in drive or overdrive.

Ignition System
    • Test for proper ignition voltage output using the following steps:
        1. Clip the J 26792 Spark Tester to engine ground.
        2. Connect one end of a spark plug wire to the J 26792 . Connect the other end of the spark plug wire to the coil being tested.
        3. Connect another spark plug wire between the remaining tower of the coil being tested and ground.
        4. Crank the engine while observing the J 26792 . A crisp blue spark should be observed.
        5. Repeat the above steps for each coil.
    • If spark is not present at the coils, inspect for the following conditions:
       - Coils--Cracks, carbon tracking/arcing, or a secondary resistance value outside the specified range

Coil Resistance
5,000-8,000 ohms (5K-8K ohms)

   Important: Spraying the secondary ignition wires with a light mist of water may help locate an intermittent condition. Ignition components will arc to ground when a secondary component is faulty.

       - Spark plug wires--Signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range

Spark Plug Wire Resistance Regular
9,840 ohms per meter (3,000 ohms per foot)

Spark Plug Wire Resistance Police
1,968 ohms per meter (600 ohms per foot)

       - Defective module
       - Ignition system wiring--Loose ignition module feed or ground connection, or damaged system wiring
    • Remove spark plugs and inspect for the following:
       - Fouled plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned or damaged electrodes
       - Improper heat range or reach
    • If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

Engine Mechanical
    • Test and inspect engine mechanical for the following:
       - Test compression--Refer to Engine Compression Test in Engine Mechanical - 3.8L.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods--Refer to Valve Rocker Arm and Push Rod Cleaning and Inspection in Engine Mechanical - 3.8L.
       - Worn rocker arms--Refer to Valve Rocker Arm and Push Rod Cleaning and Inspection in Engine Mechanical - 3.8L.
       - Broken valve springs--Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical - 3.8L.
       - Excess oil in combustion chamber--Inspect for leaking valve seals.
    •  Inspect for incorrect engine parts, such as the following basic components:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Symptoms - Engine Mechanical in Engine Mechanical

Additional Inspections
    • Test the exhaust system for possible restrictions. Inspect for the following conditions:
       - The exhaust system for damaged or collapsed pipes
       - The mufflers for heat distress or possible internal failure
       - For possible plugged 3-way catalytic converter--Refer to Restricted Exhaust in Engine Exhaust.
    • Inspect for faulty motor mounts.
    • An engine miss condition can be caused by electromagnetic interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a condition exists, inspect the routing of secondary ignition wires, high voltage components (near ignition control circuits).
    • Inspect the intake and exhaust manifold passages for casting flash.