What causes unstable idling in the Corolla 1.2T?

[HazelRose]

Unstable idling in the Corolla 1.2T is primarily caused by severe carbon buildup in the engine. Reasons for engine carbon buildup: When excessive carbon deposits accumulate inside the engine, the gasoline injected by the cold-start fuel injector gets largely absorbed by the deposits. This results in an overly lean air-fuel mixture during cold starts, making ignition difficult. Under these conditions, the engine only starts easily after the carbon deposits become saturated with gasoline. Once started, the gasoline adsorbed on the carbon deposits gets drawn into the combustion chamber by the engine's vacuum suction, making the mixture too rich. This fluctuation between lean and rich air-fuel mixtures causes unstable idling after cold starts. Reasons for excessive carbon deposits in throttle body and intake tract: When excessive carbon deposits and dirt accumulate in the throttle body and surrounding intake passages, the cross-sectional area of the air passage changes. This prevents the control unit from precisely regulating idle air intake, resulting in either overly rich or lean mixtures that lead to abnormal combustion.