GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1997 🢒 Sun Runner - 4WD (Canada) 🢒 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
    • Check the Heated Oxygen Sensor (HO2S) sensor 1 and 2. Refer to DTC P0131 through DTC P0141.
    • Check the Throttle Position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will causes 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, 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 (41°F) higher or lower than the ambient air temperature, check for a high resistance in coolant sensor circuit or the sensor itself. Refer to DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage .
    • Check the MAP sensor for a quick response and accuracy.

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 greater than 20%. Refer to DTC P0171 Fuel Trim System Lean Diagnostic Aids.
       - Rich - The Long Term Fuel Trim will be less than -20%. Refer to DTC P0172 Fuel Trim System Rich Diagnostic Aids.
    • Check the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .
    • Check the Evaporative Emission (EVAP) control system. Refer to Evaporative Emission Control System Diagnosis .
    • Check the fuel injectors for leaking. Refer to Fuel Pump Electrical Circuit Diagnosis .

Ignition System Checks
    • Check for the proper secondary voltage output with a spark tester (J 26792 or 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.
       - Platinum pads that are missing.
    • Check the spark plug cables by connecting an ohmmeter to the ends of each cable. Replace any spark plug cable(s) that reads over 30,000 ohms.
    • Check the ignition timing. Refer to Ignition Timing Procedure.

Additional Checks
    • Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.
    • Check the PCM grounds for clean, tight, and proper connections. Refer to PCM Wiring Diagram.
    • Check the scan tool to determine if the PCM is receiving an A/C signal. Idle speed should be increased with the A/C ON.
    • Check the EGR Valve for excessive flow or sticking open. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
    • Check the battery cables and ground straps. They should be clean and secure.
    • Check the A/C refrigerant pressure for being too high or for a faulty high pressure switch.
    • Check the IAC system for proper operation.
    • 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 .

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