The 1991 S10 was a transition year. The early S10 without CTF had a THERMAC and the late S10 with CTF did not have a THERMAC and the TB did not have a vacuum port nipple for the THERMAC. The air cleaner on the late S10 with CTF did not have a THERMAC.Here's a 1991 vacuum diagram:
Mine is a 91, but this is the diagram I have (with the LLW code):
The idle will always pick up with a vacuum leak, since the MAP voltage will change and fool the ECM to thinking the motor is under a load. If you have a vacuum-operating THERMAC air cleaner, then that's where it would go to. Mine doesn't use a vacuum line, but I have an 360 open-element K&N anyhow.