Air Control Valve
This valve contains 2 relief valves to control passage of air to exhaust manifold. See Fig 2. The No. 1 relief valve is closed during high load condition. As exhaust gases increase in exhaust system, part of air pump air is allowed into exhaust manifold to allow proper burning of emissions. Too much added air would create abnormally high exhaust temperatures.
The No. 2 relief valve modulates the addition of air according to operating conditions. It is directly controlled by intake manifold vacuum. When vacuum is high (deceleration) valve opens to divert part of air pump air back to air pump. This prevents afterburning caused by too much air in exhaust system.