Please Help!

I got a big problem I was doing a tune-up when I realized that the vaccum line that comes from the very back of the tbi was disconnected I'm not sure where it goes. I'm hoping I can just cap it right?!?

You dont have a manual? I think that would the egr. If not then maybe the map sensor. Im trying to remember what it all looked like on my 89.

manual doesnt say much I know it not the one going to the egr though.

Could be the one providing vacuum to your ac components and....something else but i cant remember. should go to a check valve and then split off.