What causes unstable idle speed in a car?

[Sienna]

The reasons for unstable idle speed in a car include: 1. Severe carbon buildup in the engine: Carbon buildup in the engine is the main cause of car shaking. When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold start injector will be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during startup, which can cause sudden stalling. Solution: Clean the fuel system, check the idle speed control valve, and clean the throttle body. 2. Malfunction in the ignition system: Spark plugs are wearable parts and need to be replaced regularly. Solution: Replace the spark plugs. 3. Issues with fuel supply pressure and the intake pressure sensor: If the fuel pump's supply pressure is abnormal or the intake pressure sensor's readings are incorrect or malfunctioning, it can cause the car to shake or even stall. Solution: Inspect and repair the relevant components.