Mike is correct, also called a "Decel Valve" and it's purpose if keep from backfiring upon deceleration. My S also did not have one but in order to comply with the Calif Smog Nazis, I got a good used one from 928 Intl and installed it.
That is a "huge" vacuum leak spot so if your going to ghetto plug it. Use new rubber/silicone pieces.
Vacuum Limiter (Decel Valve) Vac Route?
__________________
1981 Porsche 928 "Euro" Auto Gunsmoke Metallic Flat - Black Interior
1983 Porsche 928S "US" Auto Light Bronze (Copper) Metallic - Brown Interior **SOLD**
