Why does the engine idle roughly when cold but smooth out when warm?

[StDelaney]

The reason for rough idling when cold that improves when warm is: Under cold operating conditions, the engine oil viscosity is relatively high, resulting in increased engine running resistance and insufficient cylinder pressure, making it difficult for individual cylinders to ignite and perform work, leading to misfires. As the engine runs and warms up, the operating temperature gradually rises, improving oil fluidity. At this point, the misfiring cylinders can resume normal ignition, naturally reducing the vibration. Cold idle roughness that disappears when warm may also be caused by inaccurate air-fuel mixture ratios, occurring in both open-loop and closed-loop control systems. If the vehicle's computer makes incorrect judgments, errors in controlling the air-fuel mixture and ignition timing through actuators may occur, reducing power output and causing vibration.