When the throttle valve closes at high engine speed, a strong vacuum is generated inside the intake manifold. Some of the fuel adhering to the internal walls of the manifold evaporates, causing the air- fuel mixture to become too rich temporarily. At the same time, since the intake air volume decreases, the incomplete combustion or misfiring occurs, and a great amount of incombustible gas is emitted in the exhaust. To prevent the incomplete combustion or misfiring, the dash pot is used to keep the throttle valve from closing too suddenly.
During deceleration, the link of throttle valve touches the dash pot. Therefore the throttle valve closes slowly according to the resistance of air passing through. When the throttle valve is opened, the dash pot returns to its original position.
Deceleration Fuel Cut-off System
This system stops the fuel injection during the deceleration part of driving, and decreases the amounts of CO and HC.
Also it prevents after-burning in the exhaust pipe, and effectively decreases the amount of the fuel consumed during deceleration.
The engine ECU stops the fuel injection of the injector according to the engine speed and throttle valve opening.