F 250 4WD Pickup L6-300 4.9L (1982)
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 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 normally open timed bypass valve with vacuum vents, Fig. 86, provides a timed air dump during deceleration and when differences in vacuum
pressure between the signal port and vent port occur. The signal port must have 3 inches Hg more vacuum than the vent port to prevent the valve from
dumping. The valve is used to protect catalyst overheating.
The normally open timed bypass valve without vacuum vents supplies air to the exhaust system with a higher than normal vacuum signal during cold
start, short idles is applied, pump air is dumped through the silencer ports of the valve, Fig. 87.
