Thats the EVAP charcoal canister, its EVAP solenoid valve should be CLOSED with key off
There will be a hose from the canister to the intake manifold, solenoid valve will be on this hose, but......................
(other hose on canister runs to the top of the gas tank)
In 1991 there may not be an EVAP solenoid, if the hose from the canister runs to a port IN FRONT of the throttle plate then no solenoid since it is using PORTED VACUUM not intake vacuum
Often the throttle body will have a port on the under side, in front of throttle plate, for the EVAP canister
"In front of throttle plate" means between throttle plate and air cleaner, which is called ported vacuum area, it has very little vacuum with throttle closed and vacuum increases as throttle opens, so kind of the opposite of regular engine vacuum.
"Behind throttle plate" means between throttle plate and intake valves, so regular vacuum, highest with throttle closed lower as throttle opens
So if canister is using Ported Vacuum then yes canister should leak smoke because its an open hose from throttle body to canister, same as air cleaner would leak smoke
Ported vacuum leak can't cause high idle.