Pulse-Air System
When the engine is idling, fresh air is drawn from the air cleaner through the air shut-off valve and the aspirator valve. Exhaust pulses cause a vacuum behind the aspirator valve which draws in this fresh air. The air is injected behind the exhaust valves in the cylinder head.
The fresh air combines with the exhaust mixture and passes through the primary catalyst. The oxygen sensor measures this exhaust gas mixture and reads the mixture as "lean" since there is more air than should normally be present. The electronic control unit richness the mixture to compensate, which enables the engine to run smoothly at idle.
When the throttle is moved off idle, vacuum is present at the signal port, which closes the air shut-off valve. Since additional air is no longer present, the control unit leans the mixture to the proper ratio.