While looking over my 88 iroc with TPI 305 i just bought i noticed a vacum line with a screw in the end of it. It was lying under the throttle body,i dont see a place to hook it back up,there is a line on each bib i looked at,including the one under th T.B. I followed it back and it goes into the firewall into the car with another line that is hooked up. Its one of those small,maybe 1/8 in. hard plastic lines.I know the air ducts work on vacum so i tried them out,and the defroster comes on and blows,and the dash duct blows,but i dont get any heat to come out when i slide the **** over to heat.The vacum diagram only shows the hoses related to emissions.Any one know where i might find one with all the hoses,or know what this line is ,and where it goes?

 

Look at the pics for a heater diverter valve - then see if yours has a vacuum line to it.

 

This diagram shows the line under the plenum supplying vac to the vac pots on the tank purge system
http://www.chevythunder.com/fuel%20i...INE%20DIAGRAMS