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For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Prizm 🢒 Engine 🢒 Engine Electrical 🢒 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 - Engine Controls .

Sensor Checks
    • Inspect the heated oxygen sensors HO2S 1 and HO2S 2 for a condition that can affect sensor activity and response. Refer to Diagnostic Aids for the following DTCs:
      •  DTC P0130
      •  DTC P0133
      •  DTC P0135 or P0141
      •  DTC P0136
    • Inspect the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will causes a high TP sensor voltage. Under these conditions the PCM may not control the idle. Monitor the TP sensor angle with the scan tool. The indicated angle should be approximately 11 percent with the throttle closed.
    • Inspect the engine coolant temperature (ECT) sensor. A fixed or inaccurate ECT sensor reading can cause the engine idle to surge or race. Using the scan tool, compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is 5°C (9°F) more than or 5°C (9°F) less than the ambient air temperature, inspect the resistance of the coolant sensor. Refer to Temperature Versus Resistance . If the engine coolant temperature is significantly less than the ambient air temperature, inspect the coolant sensor electrical circuit for high resistance.
    • Inspect the mass air flow (MAF) sensor operation. Run the engine to normal operating temperature. Turn OFF all accessories and observe the MAF sensor at idle with a scan tool. If the MAF sensor reading is more than 3.5 g/s refer to DTC P0101 .

Fuel System Checks
    • Inspect the operation of the fuel system for rich or lean condition. Operate the vehicle under the conditions that caused the concern. Monitor the Fuel Trim parameter on a scan tool in order to identify the problem.
       - Lean -- The Long Term Fuel Trim will be more than 20 percent. Refer to Diagnostic Aids in DTC P0171 .
       - Rich -- The Long Term Fuel Trim will be less than -20 percent. Refer to Diagnostic Aids in DTC P0172 .
    • Test the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .
    • Inspect the evaporative emission (EVAP) control system. Refer to Evaporative Emission Control System Diagnosis .

Ignition System Checks
    • Test for the proper secondary voltage output with a J 26792 .
    • Inspect the spark plugs for correct operation and good ignition system performance. Refer to Spark Plug Inspection .
    • Inspect the ignition coil boots for damage.
    • Observe the IAC Motor Command parameter on the scan tool at idle, while operating the rear defogger, the A/C system, or the headlights. The IAC Motor Command parameter should increase 2-7 percent when operating the engine loads. If the IAC Motor Command parameter remains fixed, check for a short to ground in the Diagnostic Request circuit from the PCM to the TE1 terminal of the underhood diagnostic link connector.

Engine Mechanical Check
    • Inspect for any of the following engine mechanical problems:
       - Any faulty hydraulic lifter assemblies
       - 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 the Diagnostic Starting Point - Engine Mechanical in Engine Mechanical.

Additional Checks
    • Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.
    • Check the scan tool to determine if the PCM is receiving an A/C signal. The idle speed should be increased with the A/C ON. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Inspect the battery cables and the ground straps. They should be clean and secure.
    • Inspect the A/C refrigerant pressure for being too high or for a faulty high pressure switch. Refer to VDOT Air Conditioning (A/C) System Diagnosis .
    • Test for the proper operation of the IAC system. Refer to Idle Air Control (IAC) System Diagnosis .
    • The engine will run rough and the engine can stall if the CMP actuator solenoid is advancing camshaft timing at idle. Inspect for a fully energized or stuck CMP actuator solenoid. If the operation of the CMP actuator solenoid is suspect, refer to DTC P1349 .
    • Inspect the PCV valve for correct operation and the correct part. Refer to Crankcase Ventilation System Inspection in Engine Mechanical.
    • Inspect that the idle speed is increasing for changes in electrical loads on the engine. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Verify that the PCM is increasing the idle speed when the P/S system pressure is increased on turns. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
    • Inspect for broken or worn motor and transmission mounts.