E 350 Van L6-300 4.9L VIN E 1-bbl (1982)
The normally closed timed bypass valve, Fig. 73, is used on vehicles with catalytic converter. During normal operation, engine manifold vacuum
applied through the vacuum differential valve (VDV) holds the valve upward, allowing thermactor air to flow to the exhaust manifolds and
blocking the vent port. When intake manifold vacuum rises or drops sharply, the VDV operates and momentarily cuts off the vacuum to the bypass
valve.
The spring pulls the stem down, seating the valve to cut off pump air to the exhaust manifolds and dumping the dump valve to momentarily divert
the air to atmosphere. If air pump pressure becomes excessive or there is a restriction in the system, the excess pressure will unseat the valve in the
lower portion of the bypass valve and allow a partial flow of pump air to the atmosphere, while at the same time, the valve in the upper part of the
bypass is still unseated, allowing a partial flow of pump air to the exhaust manifold to meet system requirements.
The timed air bypass valve with an integral vacuum differential function, Fig. 74, is used on some thermactor systems. This valve contains an
integral vacuum differential function, therefore a separate vacuum differential valve (VLV) is not required. When the vent is blocked, the valve
functions as a timed bypass valve with a VDV. However, when the vent is open to atmospheric pressure, and a vacuum of 4 inches Hg or more is
applied to the source port, the valve goes to the dump mode.
