California Models (Over 10,000 GVW)
Under normal vacuum conditions, metering valve is held open by a spring on vacuum side of diaphragm. This allows air from engine to pass into exhaust manifold. During deceleration (very high vacuum), vacuum is applied to diaphragm to overcome spring tension, closing metering valve momentarily. During closed metering valve, air is diverted to air cleaner or silencer. See Fig 1 .