The last time I had the air filter box off I noticed a small diameter hose dangling down the fire wall. The hose appears to be a vacuum hose and exits the firewall just under the heater valve hose (red hose of small diameter). I have not noticed any significant idle roughness thus far. I used a flash light and searched high and low for the connect point, but success was elusive.

While driving the car this morning I attempted to engage the cruise control and got nothing. The last time I used the car it worked and, coincidentally, this was before I performed some engine cleaning, plug and plug wire change. I must have knocked the hose loose during the process and strongly suspect it belongs to the cruise control.

Can someone give me a hint as to where to look for the connect point. I have a '79 928 5-spd.

 

The cruise control vacuum attaches thru the fender well, to the four way vacuum connector by the brake booster. Sorry, no picture. I am too busy right now. But if you want, later I can return with a photo. Also you might try running the car, shutting it off, and listening for vacuum leaks. By the way, I'm a rookie 928 owner myself, so sometimes my advice may be WRONG!