What are the reasons for unstable idle speed when starting the car?

[VanRonan]

There are mainly five reasons for unstable idle speed when starting the car, which are: 1. Excessive carbon deposits in the engine. When there is too much carbon buildup inside the engine, the gasoline sprayed by the fuel injector during startup will be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture and causing difficulty in starting. The vehicle will only start easily once the carbon deposits have absorbed enough gasoline to reach saturation. After the engine starts, the gasoline attached to the carbon deposits will be sucked into the cylinders by the engine's vacuum, making the air-fuel mixture too rich. The fluctuating richness of the air-fuel mixture causes idle speed fluctuations during startup. 2. Air filter. If the air filter is not replaced in time, it can lead to insufficient air intake, causing the engine idle speed to drop. 3. Abnormal water temperature. High engine water temperature can cause early ignition timing, alter the air-fuel mixture ratio, and thin the engine oil viscosity (reducing lubricity). Causes of abnormal water temperature include: insufficient coolant, a stuck thermostat, fan malfunction, or inaccurate temperature control switch. 4. Excessive carbon buildup in the throttle body or mass air flow sensor. The throttle body and mass air flow sensor are prone to getting dirty. Most modern cars use electronic throttle bodies, and even a small amount of carbon buildup can affect performance. Excessive carbon deposits in the throttle body can cause sluggish acceleration, severe power loss, and unstable idle speed. 5. Clogged fuel injectors. Clogged fuel injectors can lead to inconsistent fuel spray among the injectors, causing uneven cylinder performance and resulting in unstable idle speed during ignition.