Rough, Unstable, or Incorrect Idle and Stalling
WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Rough, Unstable, or Incorrect Idle and 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. |
| Preliminary Checks |
Refer to Important Preliminary Checks in Symptoms - Engine Controls
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| Sensor Checks |
- Test for proper operation and performance of the oxygen sensor.
Inspect the oxygen sensor for silicon contamination. Caused by fuel or the use of improper engine sealants, the oxygen sensor will have a white, powdery coating. The sensor will output a high, but false signal voltage. This false rich condition may cause the PCM to reduce the amount of fuel to the engine.
- Test the ECT sensor for proper operation by performing the following:
- Allow the engine temperature to reach ambient air temperature by leaving the engine OFF for several hours - overnight for most accurate results.
- Turn the ignition ON while leaving the engine OFF.
- Using a scan tool compare the ECT and IAT sensors displayed temperature.
The sensors should be within 3° C (5° F) of each other.
- Replace the ECT sensor or repair the ECT sensor circuit if the ECT is not near the expected value.
- Test for proper MAP sensor operation. A MAP sensor that does not respond quickly and accurately to manifold pressure changes may cause this symptom.
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| Fuel System Checks |
- Determine if the condition is rich or lean by performing the following:
- Operate the vehicle under the conditions of the concern.
- Monitor the oxygen sensor output.
- Test for leaking fuel injectors.
- Inspect for fuel - gasoline - in the fuel pressure regulator vacuum hose, if equipped. This indicates a faulty gasoline fuel pressure regulator.
- Inspect for proper operation of the CNG fuel injectors. Refer to:
- Test for proper EVAP Purge system operation.
- Test for proper alternative fuel system pressures. Refer to Fuel System Diagnosis
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| Additional Checks |
- Inspect for manifold or air induction system leaks.
- Test the PCV system for proper operation and valve type.
- Test for proper engine cylinder compression.
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