GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1998 🢒 Metro 🢒 Engine 🢒 Engine Controls - 1.3L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

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DEFINITION: The engine runs unevenly at idle. The engine or vehicle may shake if the condition is severe enough. The engine idle speed may vary in RPM. The condition may be severe enough to stall the engine. The engine idles at an incorrect speed.

Preliminary Check
    • Refer to Symptoms .

Sensor Checks
    • Check the performance of the heated oxygen sensor (HO2S) sensor 1 and the HO2S 2. Refer to Diagnostic Aids in DTC P0131 through DTC P0141.
    • Check the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will cause a high TP sensor voltage (open throttle indication). Under these conditions the PCM may not control the idle. Monitor the TP sensor voltage with the scan tool or a voltmeter. The indicated voltage should be less than 1.25 volts with throttle closed.
    • Check the engine coolant temperature (ECT) sensor. Use a scan tool in order to compare the engine coolant temperature reading on a cold engine with the ambient air temperature reading of the same engine. A coolant temperature reading that is more than 5°C (41°F) different than the ambient air temperature reading (on a cold engine) may indicate a high resistance in the coolant sensor circuit or the sensor itself. Refer to Temperature vs Resistance .
    • Check the performance of the MAP sensor. Refer to DTC P0106 Manifold Absolute Pressure (MAP) System Performance Diagnostic Aids.
    • Check the CKP sensor engine reference signal with a scan tool. Observe the Engine Speed parameter while running the engine at idle. The scan tool should indicate steady and linear RPM readings while running. If the erratic values, such as sudden spikes or drops in engine speed are displayed, the engine reference signal is not stable enough for the engine to run properly. Check for a short to ground in the CKP sensor Ignition Reference Low circuit . A short to ground in the CKP sensor Ignition Reference Low circuit will cause a faulty engine reference signal to be sent to the PCM, but will not set a CKP sensor DTC (P0335).

Fuel System Checks
    • Determine whether a rich or lean fueling condition can cause the concern. Drive the vehicle at the speed of the concern. Monitor the Fuel Trim parameters on a scan tool in order to identify the fuel system status.
       - Lean -- The Long Term Fuel Trim value will be approaching 20%. Refer to DTC P0171 Fuel Trim System Lean Diagnostic Aids.
       - Rich -- The Long Term Fuel Trim value will be approaching -20%. Refer to DTC P0172 Fuel Trim System Rich Diagnostic Aids.
    • Check for a faulty, leaking, or improperly operating fuel injector. Perform the Fuel Injector Coil Test Procedure and the Fuel Injector Balance Test Procedure. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and Sequential Multiport Fuel Injector Balance Test .
    • Check the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .
    • Check the EVAP control system. Refer to Evaporative Emission Control System Diagnosis .

Ignition System Checks
    • Check for adequate secondary ignition voltage with a J 26792 spark tester or an equivalent.
    • Check the spark plugs for any of the following conditions:
       - Wet (fuel fouled) plugs.
       - A terminal screw that is bent or loose (rotate and pull).
       - A ceramic insulator that has cracks, carbon tracking or deposits (red or black dust)
       - A tip insulator that has cracks, carbon tracking, deposits (oil, carbon, glazing, fuel additive), or is loose (rattles).
       - A center electrode that is loose (wiggles side to side)
       - An incorrect spark plug gap that is too large or too small.
    • Check the spark plug cables by connecting an ohmmeter to the ends of each cable. Replace any spark plug cable that reads over 30,000 ohms.

Engine Mechanical Check

Check for any of the following engine mechanical concerns:

    • Incorrect valve lash
    • Broken or weak valve springs
    • Low compression
    • Incorrect valve timing
    • Sticking or leaking valves
    • Incorrect or worn camshaft

Refer to Engine Mechanical.

Additional Checks
    • Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.
    • Check that the battery cables and ground straps are clean and secure.
    • Check the PCM grounds for being clean, tight, and in the proper location. Refer to PCM Wiring Diagrams.
    • An idle concern that occurs when the A/C is ON may indicate a fault in the A/C idle-up system. Idle speed should increase with the A/C ON. Check the scan tool in order to determine if the PCM is receiving an A/C signal. Refer to Air Conditioning (A/C) Idle Circuit Diagnosis .
    • Inspect the engine for the correct idle speed. Refer to Idle Speed Inspection .
    • A rough running or stalling condition that occurs when the A/C is ON may be caused by A/C refrigerant pressure that is too high or a faulty high pressure switch.
    • Check the idle air control (IAC) system for proper operation. Use a scan tool in order to command the IAC valve to increase/decrease engine idle speed.
    • Check the PCV valve for the correct part and proper operation. Refer to Crankcase Ventilation System Inspection .
    • Check that the idle speed is adjusted for increased electrical loads. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Check the PSP switch for proper operation. Refer to PSP Switch Diagnosis .