Deceleration Mode

When the driver releases the accelerator pedal, air flow into the engine is reduced. The corresponding changes in throttle position and manifold air pressure are relayed to the Electronic Control Module (ECM), which reduces the injector pulse width, to reduce fuel flow. If the deceleration is very rapid, or for long periods (such as long closed throttle coast-down), the ECM shuts fuel completely "OFF," to protect the catalytic converter.