Suburban 3/4 Ton 4WD V8-305 5.0L (1986)
Fig. 80 Deceleration valve
The PAIR system, Fig. 79, is used on some vehicles and consists of pulse air check valves which are connected by tubes to the exhaust ports. As the
engine fires, a pressure, either positive or negative, is created. When the pulse air valve receives a negative pressure, fresh air flows into the exhaust
system. A positive pressure will force the check valve closed, stopping exhaust gas flow past the valve and into the fresh air supply line.
A deceleration valve, Fig. 80, used on some engines, prevents backfiring in the exhaust system during deceleration. When deceleration causes an
abrupt vacuum increase in the vacuum signal line, the pressure differential on the diaphragm overcomes spring tension, opening the valve and bleeding
air into the intake manifold. Air trapped in the chamber above the vacuum diaphragm will be bled at a calibrated rate through the delay valve portion of
the integral check and delay valve, reducing vacuum on the diaphragm. When vacuum on the diaphragm and spring tension become equal, the valve
closes and cuts off air supply to the intake manifold. The check valve portion of the check and delay valve provides a rapid balancing of chamber
pressure when there is a sudden decrease in vacuum during acceleration.
