What causes the idle pointer to float up and down?

The tachometer on the dashboard of the vehicle can let the driver know the current engine speed, but sometimes there may be some abnormalities in the tachometer on the vehicle, such as the pointer of the tachometer floating up and down. So what is the reason why the idle pointer floats up and down?

Idle pointers float up and down for two reasons:

If the idle pointer floats up and down when the car is cold: there may be carbon deposits in the intake system, so it is generally necessary to check whether there is too much carbon deposits in the throttle, whether the idle valve and the oil pump are damaged, and the signal feedback from the water temperature sensor is disordered, which will also lead to slight jitter of the speed pointer when the engine is idling, because the idle speed of the engine is automatically controlled by ECU.

If the idle pointer floats up and down when the car is hot, it may be caused by air leakage from the waste gas valve, lax sealing of the throttle valve, and abnormal pressure of the spark plug and cylinder. This situation is unstable in various engine operations, and misfires occur in one or more cylinders. It is recommended to check whether the spark plug and throttle valve work normally before cleaning.