What's the matter with the increase of gear idle speed when Jetta is driving?

As for the analysis of the situation you reflected, the reason may be that the idle speed control device is stuck or beyond its adjustment range because of the carbon deposit blocking the idle speed passage, or that the throttle valve is stuck because of too much carbon deposit, which leads to the increase of idle speed after receiving oil. In many cases, after cleaning the throttle, the idle speed of oil collection will increase. This is because when cleaning the carbon deposit, the carbon deposit solution enters the throttle shaft, causing the blockage between the throttle shaft and the valve body base. Sometimes it is serious to replace the throttle body.

According to the above analysis, it is recommended that you go to a 4s shop or a regular repair shop for on-site inspection.

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