GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Tracker - 2WD 🢒 Engine 🢒 Engine Controls - 2.0L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

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 mass air flow (MAF) sensor. Refer to DTC P0101 Mass Air Flow (MAF) Sensor Performance Diagnostic Aids.
    • Check the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will cause a high TP sensor voltage or an open throttle indication. Under these conditions the powertrain control module (PCM) may not control the idle. Monitor the TP sensor voltage with the scan tool or a digital multimeter. The indicated voltage should be less than 1.25 volts with the throttle closed.
    • Inspect for a shifted or fixed engine coolant temperature (ECT) sensor. Test the ECT sensor using the scan tool. Compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is more than 5°C (9°F) more or less than the ambient air temperature, check for a high resistance in the coolant sensor circuit or the sensor itself. Refer to Temperature Versus Resistance .
    • Inspect the performance of the heated oxygen sensors (HO2S) 1. Refer to:
    • Check the camshaft position (CMP) 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.

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 percent. Refer to DTC P0171 Fuel Trim System Lean Diagnostic Aids.
       - Rich -- The long term fuel trim value will be approaching -20 percent. 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 Fuel Injector Balance Test .
    • Check the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .
    • An EVAP purge valve that allows excess purge at idle can cause a rich fueling condition that affects engine performance. 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:
       - Any wet plugs
       - A terminal screw that is bent or loose
       - A ceramic insulator that has cracks, carbon tracking or deposits
       - A tip insulator that has cracks, carbon tracking, deposits, or is loose
       - A center electrode that is loose
       - An incorrect spark plug gap that is too large or too small
    • Check the ignition coil and spark plug boots for cracks, carbon tracking, and moisture.
    • Check for the correct ignition timing. Refer to Ignition Timing Adjustment .

Engine Mechanical Check

Check for any of the following engine mechanical concerns:

    • An incorrect valve lash
    • Any broken or weak valve springs
    • A low compression
    • An incorrect valve timing
    • Any sticking or leaking valves
    • An incorrect or worn camshaft

Refer to Engine Compression Test in Engine Mechanical.

Additional Checks
    • Check for vacuum leaks. Vacuum leaks can cause higher than normal and erratic idle speeds.
    • Check the air inlet system for any leaks or restrictions.
    • Check the throttle body for blockage and a damaged or binding throttle valve.
    • Check for binding of the accelerator cable or pedal. Inspect the accelerator cable for the proper adjustment. Refer to Accelerator Cable Adjustment .
    • Verfiy that the battery cables and ground straps are clean and secure.
    • Inspect the exhaust gas recirculation (EGR) valve for leaks and proper operation. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
    • Check for a DTC C1215 in the ABS idle-up circuit. Refer to DTC C1215 Idle Up Circuit Malfunction .
    • Check the idle air control (IAC) system for proper operation. Use a scan tool in order to command the IAC valve to increase and/or decrease the engine idle speed. Refer to Idle Air Control (IAC) System Diagnosis .
    • An idle concern that occurs when the A/C is On may indicate a fault in the A/C idle-up system. The 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 .
    • 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.
    • The PCM is designed to respond to changes in the engine load. Inspect the electrical load idle-up system for proper operation. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Inspect the throttle body and the IAC valve for excessive carbon deposits. A buildup of deposits in the passages, on the IAC pintle, and around the throttle valve and the throttle bore can affect IAC system performance. Remove the excess deposits as necessary. Refer to Throttle Body Inspection and Cleaning .
    • Check the positive crankcase ventilation (PCV) valve for the correct part and proper operation. Refer to Crankcase Ventilation System Inspection .
    • Check the power steering pressure (PSP) switch for proper operation. Refer to PSP Switch Diagnosis .
    • Check for proper adjustment of the transmission range (TR) switch. Observe the park/nuetral position (PNP) parameter on the scan tool for the correct position display, when shifting the transmission thru the PRNDL pattern. Service the TR switch as necessary. Refer to DTC P0705 Trans Range Switch Circuit .
    • Inspect the engine cooling system for proper operation. An engine overheating condition or the failure of the engine to reacha normal operating temperature may affect the engine idle speeds. Refer to Engine Overheating and Engine Fails To Reach Normal Operating Temperature in Engine Cooling.
    • A malfunctioning air limiting valve in the throttle body could cause the following conditions:
       - A slightly higher than normal idle speed on a hot engine when the air limiting valve is stuck open.
       - A lower than normal idle speed on a cold engine when the air limiting valve is stuck closed.
    • If either condition is present and the IAC system is OK, replace the air limiting valve. Refer to Throttle Body Assembly Replacement .