What Causes Unstable Idle and Automatic Stalling?

[DeMariana]

Unstable idle and automatic stalling can be caused by the following reasons: Severe Engine Carbon Buildup: When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold start injector is largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts and making it difficult to start the engine. In this situation, the engine only starts easily once the carbon deposits have absorbed enough gasoline. After starting, the gasoline adsorbed on the carbon deposits is drawn into the combustion chamber by the engine's vacuum suction, enriching the air-fuel mixture again. This fluctuation between lean and rich mixtures causes unstable idle and shaking after a cold start. Excessive Dirt in Throttle Body and Intake Manifold: When there is too much carbon buildup or dirt in the throttle body and surrounding intake passages, the cross-sectional area of the air passage changes, making it difficult for the control unit to precisely regulate the idle air intake. This leads to an overly rich or lean air-fuel mixture, resulting in abnormal combustion.