My '98 XJ8, new cam chains/tensioners/guides.
I don't know where this tiny plastic tube goes. It comes from this sensor assembly (?) With the 2 yellow caps,, next to the brake booster. It must tie into the vacuum system. Can I tee it into a common vacuum line?

 

There should be a vacuum reservoir under the air filter housing.
All part of the AJ26 speed control.

 

You're right, one line does go under the air cleaner.. That was already connected,, but I found the unknown one had broken free from the tee connection beneath the sensor just behind the throttle body. I used some clear tubing from Lowes to make the broken connections in a couple places. See pic. Thank you!