Some vacuum advances have two vacuum ports, one for a vacuum advance and one for a vacuum retard. The retard one is usually blocked off with a cap like you describe, so are you sure there is not another port that is either there or has been broken off.New (to me) 1994 Disco. The vacuum advance hose comes out of the plenum but is just cut off. I bought new hose tonight to put on tomorrow. The port (on the distributor) where I'm supposed put the hose actually has cap on it.... My guess is that the distributor was replaced at some point and the hose was never installed. The truck doesn't idle right when its warm. Likes to surge then drop to the point where it stalls. Would this unattached hose cause this? (vacuum leak, vacuum advance, etc.) Any thoughts would be helpful. Thanks in advance.