Why is the car idle unstable?

[Olga]

There are several reasons for unstable car idle, including oil accumulation and carbon deposits in the throttle body, as well as dirty or clogged intake passages, which can cause idle fluctuations. Carbon deposits or oil in the throttle body are the most common causes of unstable idle. Below are some related reasons for unstable car idle: Spark plug failure: Weak sparks from faulty spark plugs can also lead to carbon buildup. Carbon deposits can be cleaned to restore spark plug activity, but some spark plugs may be nearing the end of their lifespan, resulting in reduced performance. Fuel system blockage: Blockages in the fuel system, particularly in the fuel injectors, or faults in the intake manifold can cause insufficient fuel supply pressure, leading to poor fuel atomization. This disrupts the air-fuel mixture ratio, resulting in incomplete combustion and related issues. Sensor failure: Faulty sensors can prevent the ECU from accurately controlling the air-fuel ratio. Common sensor failures include the coolant temperature sensor, oxygen sensor, and throttle position sensor. The coolant temperature sensor is typically installed on the engine block or cylinder head water jacket, directly contacting the antifreeze to measure temperature and provide reference data to the ECU.