Here's why:
* Heat Soak: The idle air control motor is a small electric motor that regulates airflow to the engine at idle. It operates continuously when the engine is running and gets quite warm. When you turn off the ignition, the engine's heat continues to radiate to the IAC motor. This heat causes the motor's internal components to expand.
* Thermal Cycling and Contraction: As the motor begins to cool after shutdown, the components contract unevenly. This uneven contraction can cause a temporary vibration or buzzing sound. The vibration isn't necessarily indicative of a malfunctioning motor, but rather a side effect of its thermal properties and rapid cooling.
* Mechanical Linkage (Less Likely): While less probable, it's possible there's a slight binding or a bit of play in the linkage connecting the IAC motor to the throttle plate. This minor mechanical imperfection could be amplified by the thermal expansion and contraction creating the vibration you're feeling.
To further diagnose, consider:
* Duration: How long does the vibration last? If it's only for a few seconds, it's very likely due to thermal effects. If it lasts longer or happens frequently, there might be an underlying mechanical issue.
* Severity: Is it a minor buzz or a strong vibration? Again, a subtle vibration is more suggestive of the thermal explanation.
* Other Symptoms: Does the car have any other issues, such as rough idle, stalling, or poor fuel economy? These symptoms could indicate problems with the IAC motor itself or other related engine systems.
If the vibration is a minor, short-lived issue and the car runs smoothly otherwise, it's probably nothing to worry about. However, if the vibration is significant, lasts a long time, or is accompanied by other symptoms, you should have your vehicle checked by a mechanic to rule out more serious problems with the IAC motor or other engine components.
