Air Bypass Valves

Fig. 57 Normally closed air bypass valves:

Normally Open Air Bypass Valves W/Vacuum Vents:

Fig. 59 Normally open air bypass valves without vacuum vents:

There are two general groups of bypass valves, normally open and normally closed. Both groups are available in both remote and pump-mounted versions.
Normally closed valves, Fig. 57, supply air to the exhaust system with medium and high applied vacuum signals during normal modes (engine operating at normal operating temperature), short idles, and some accelerations. With low or no vacuum applied to pump, air is dumped through silencer ports of valve.
Normally open valves with a vacuum vent, Fig. 58, provide a timed air dump during decelerations and also dump when a vacuum pressure difference is maintained between the signal port and the vent port. The signal port must have 3 inches Hg more vacuum than the vent port to hold the dump. This mode is used to protect the catalyst from overheating.
Normally open valves without a vacuum vent, Fig. 59, provide a timed dump of air for 1.1 or 2.8 seconds when a sudden high vacuum of about 20 inches Hg is applied to the signal port. This prevents backfire during deceleration.