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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Grand Am 🢒 Engine 🢒 Engine Electrical 🢒 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 - Engine Controls .
    • 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 with Special Tool or Ignition System Specifications .
    • 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 .
    • 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 .
    • 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 DTC P0105 .
    • Test for the correct operation of the throttle position (TP) sensor. Refer to DTC P0105 .
    • Test for the correct operation of the Idle Air Control (IAC) motor. Refer to Idle Air Control (IAC) System Diagnosis .
    • Test for the correct operation of the engine coolant temperature (ECT) sensor. Refer to DTC P0117 and DTC P0118 .
    • 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.

Ignition System
    • Test for the correct ignition voltage output with the J 36012-A spark plug jumper wires and J 26792 spark tester or equivalent.
    • 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.
       - An incorrect valve timing.
       - Any excessive oil in combustion chamber and leaking valve seals. Refer to Symptoms - Engine Mechanical in Engine Mechanical.
    • For incorrect basic engine parts. Inspect the following:
       - The camshafts
       - The cylinder head
       - The pistons, etc.
       - Refer to Symptoms - Engine Mechanical 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 in Engine 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 Symptoms - Engine Mechanical in Engine Mechanical .