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For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2000 🢒 Alero Export 🢒 Engine 🢒 Engine Controls - 2.4L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Rough, Unstable, or Incorrect Idle, Stalling

Tests/Inspection

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
    • Refer to Symptoms .
    • Search for service bulletins.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Diagnosis .
    • Test the fuel injectors. Refer to Fuel Injector Balance Test .
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect the fuel filter for obstructions.
    • Inspect the items which can cause an engine to run rich (long term fuel trim significantly in the negative range). Refer to Diagnostic Aids for DTC P0172 Fuel Trim System Rich .
    • Inspect the items that can cause an engine to run lean (long term fuel trim significantly in the positive range). Refer to Diagnostic Aids for DTC P0171 Fuel Trim System Lean .
    • Test for the correct operation of the evaporative emissions (EVAP) control system. Refer to Evaporative Emission Control System Diagnosis .
    • Inspect for fuel in the fuel pressure regulator vacuum hose.

Sensor/System
    • Test for the correct operation of the manifold absolute pressure (MAP) sensor. Refer to Manifold Absolute Pressure Sensor Output Diagnosis .
    • Test for the correct operation of the throttle position (TP) sensor.
    • Test for the correct operation of the Idle Air Control (IAC) motor.
    • Test for the correct operation of the engine coolant temperature (ECT) sensor.
    • Test the oxygen sensor (O2S 1). The O2S 1 should respond quickly to different throttle positions. If not, inspect the O2S 1 for silicon or other contaminants from fuel or the use of improper RTV sealant. The sensor may have a white powdery coating. Silicon contamination causes a high but false O2S 1 signal voltage (rich exhaust indication). The PCM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Refer to Information Sensors/Switches Description .

Ignition System
    • Test for the correct ignition voltage output.
    • Inspect the spark plugs for the following conditions:
       - Any wet or fuel fouled plugs
       - Any cracks
       - An incorrect gap
       - Any burnt electrodes or heavy deposits
    • If the spark plugs are fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Inspect the ignition control module (ICM) for a correct ground connection.
    • Inspect the ignition coils for cracks or carbon tracking.

Engine Mechanical
    • Inspect for the following:
       - A low compression. Refer to Engine Compression Test in Engine Mechanical.
       - Any broken or weak valve springs.
       - Any sticking or leaking valves.
       - Any worn camshafts lobes.
       - Refer to Valve Train Diagnosis in Engine Mechanical.
       - An incorrect valve timing. Refer to Camshaft Timing Chain and Sprocket Cleaning and Inspection in Engine Mechanical.
       - Any excessive oil in combustion chamber and leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • For incorrect basic engine parts. Inspect the following:
       - The camshafts
       - The cylinder head
       - The pistons, etc.
       - Refer to Engine Mechanical Specifications in Engine Mechanical.

Additional
    • Inspect the exhaust system for the following conditions:
       - Any damaged or collapsed pipes
       - Any heat distress or internal failure of the muffler
       - A plugged 3-way catalytic converter. Refer to Restricted Exhaust .
    • Inspect for the correct operation of the crankcase ventilation system.
    • Inspect for any malfunctioning engine mounts. Refer to Engine Mount Inspection in Engine Mechanical.
    • An engine miss condition can be caused by electromagnetic interference (EMI) on the reference circuit. EMI can usually be detected by monitoring the engine RPM with a scan tool. A sudden increase in the RPM with little change in actual engine RPM indicates EMI is present. If a problem exists, inspect the routing of secondary ignition wires near the high voltage components.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection and to Exhaust Manifold Cleaning and Inspection in Engine Mechanical .