GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2000 🢒 Tracker - 2WD 🢒 Engine 🢒 Engine Controls - 1.6L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Rough, Unstable, or Incorrect Idle, Stalling

Checks

Action

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

Preliminary Check
    • Refer to Symptoms .

Sensor Checks
    • Inspect the operation of the heated oxygen sensor (HO2S) 1. Refer to:
    • 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.
    • 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 .
    • Check the performance of the MAF sensor. Refer to Diagnostic Aids in DTC P0101 Mass Air Flow (MAF) Sensor Performance .

Fuel System Checks
    • Check if a Rich or Lean system can cause the condition. Drive the vehicle at the speed of the complaint. Monitoring the Fuel Trim may help identify the problem.
       - Lean - The Long Term Fuel Trim will be approaching 20 percent. Refer to DTC P0171 Fuel Trim System Lean , Diagnostic Aids.
       - Rich - The Long Term Fuel Trim will be approaching -20 percent. Refer to DTC P0172 Fuel Trim System Rich , Diagnostic Aids.
    • Check the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .
    • An EVAP canister purge solenoid that allows excess purge at idle, can cause a rich fueling condition that affects engine performance. Refer to Evaporative Emission Control System Diagnosis .
    • Check the fuel injectors for leaking. Refer to Fuel Injector Balance Test .

Ignition System Checks
    • Check for the proper secondary voltage output with a J 26792 spark tester or equivalent.
    • Check the spark plugs for any of the following conditions:
       - Wet (fuel fouled) spark plugs
       - A terminal screw that is bent or loose (rotate and pull)
       - A ceramic insulator that has cracks, carbon tracking, or excessive 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
       - Platinum pads that are missing
    • Check the spark plug cables at cylinders 1 and 3 by connecting an ohmmeter to the ends of each cable. Replace any spark plug cable that reads over 30,000 ohms.
    • Check for loose ignition coils connections at cylinders 2 and 4.
    • Check for cracked ignition coils boots at cylinders 2 and 4.

Engine Mechanical Check
    • Check for any of the following engine mechanical concerns:
       - Faulty hydraulic lifter assemblies
       - Broken or weak valve springs
       - Low compression
       - Incorrect valve timing
       - Incorrect or worn camshaft
    • Refer to Engine Compression Test in Engine Mechanical.

Additional Checks
    • Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.
    • Check the air inlet system (ducts, air cleaner, MAF sensor) 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 .
    • Check for clean and tight battery cables and ground straps.
    • Check the 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/decrease engine idle speed. Refer to Idle Air Control (IAC) System Diagnosis .
    • The PCM is designed to respond to changes in engine load. Inspect the electrical load idle-up system for proper operation. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Check the scan tool in order to determine if the PCM is receiving an A/C signal. Idle speed should be increased with the A/C ON. Refer to Air Conditioning (A/C) Idle Circuit Diagnosis .
    • Check the A/C refrigerant pressure for being too high or for a faulty high pressure switch.
    • Check the PCV valve for proper operation by placing a finger over inlet hole in valve end several times. The valve should snap back. Refer to Crankcase Ventilation System Inspection .
    • Inspect the engine cooling system for proper operation. An engine overheating condition or the failure of the engine to reach normal operating temperature may affect 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 .