What's wrong with Jetta's unstable idling speed?

If there is still a problem after replacing the idle motor, the fault may be caused by the increase of carbon deposition in throttle, injector, intake port, valve and combustion chamber. If carbon deposition occurs in the combustion chamber, the mixture will be too lean when the engine is idling, and too much carbon deposition in the combustion chamber will lead to the change of the compression ratio of the engine, which will lead to unstable idling. So you should check whether there is too much carbon and dirt in the air intake and intake valve. Too much carbon deposit and dirt will change the cross-sectional area of the air passage, which will make the control unit unable to accurately control the idle intake, resulting in too lean mixture, which will lead to abnormal combustion or engine jitter or even failure to start after starting. At the same time, it is also necessary to check the working state of the fuel injector to see if the pulse width of the fuel injection is up to standard. Spark plugs, ignition coils and cylinder lines are also needed, and whether there is a lack of cylinders. Remind you whether the carbon canister is out of order. You can also find time to run the expressway, which will alleviate the formation of carbon deposits.