There is some sort of solenoid or vacuum switch mounted on the firewall roughly above the valve cover. It is a metal cylinder approx. 1.25" diameter by 2" long. There are two tubes and one small electrical connector. One tube goes to the intake manifold. The other tube goes to a tee fitting which continues into the throttle body next to the black knob that adjusts idle speed on the K-jet injection system. The electrical fitting is two wires, one black and one green. The green wire disappears into a wiring bundle on one segment, there is a branched wire that ends in a spade terminal that isn't connected to anything. I search my Haynes manual and can't figure out what it is. If I disconnect the vacuum line the idle speed rises. I am trying to debug a rough idle. There is also a small white plastic cap on the end of the metal cannister. I can turn the cap but nothing seems to change when I do.

What is it?

How can I check if it is operating properly?

Is it causing my idle problems?

Should the green wire be connected?

Thanks for the help.