All 1980 models and all 1981 and later models with the V6 engine utilize the deceleration valve. Its purpose is to prevent backfiring in the exhaust system during engine deceleration. The valve is normally closed. When the throttle is suddenly closed, vacuum increases in the signal line (hose) to the valve. This opens the valve, which bleeds air into the intake manifold, leaning out the rich deceleration mixture.
Air trapped in a chamber above the vacuum diaphragm bleeds at a predetermined rate through the delay valve portion of a check and delay valve, located centrally in the diaphragm. The air bleed reduces vacuum acting on the diaphragm. When vacuum above the diaphragm falls below the level necessary to counteract diaphragm-closing spring pressure, the delay valve closes, shutting off intake air bleed.
The check valve portion of the check and delay valve balances vacuum chamber pressure when vacuum is caused by acceleration, rather than deceleration.