What causes the Jetta's idle speed to fluctuate erratically?

[GideonLee]

It is due to dirty, worn, or incorrectly installed mechanical components. Below is relevant information about idle speed: Introduction: Idle speed refers to an operating condition of a car where the engine runs in neutral gear. The engine speed during idle is called the idle speed. The idle speed can be adjusted by modifying the throttle size, among other methods. Essentially, idle speed means the engine is "producing power without doing work." Function: The idle speed can be adjusted by modifying the throttle opening size and idle fuel supply, among other factors. Generally, the optimal idle speed is the lowest speed at which the engine runs smoothly within the idle range and has good acceleration performance.