A line runs from the intake manifold  to a charcoal canister located in front of the right wheel well.  The bottom of the canister is open to the atmosphere.  When the engine is running, 12 volts is applied to a solenoid valve on top of the canister; this prevents flow from the canister to the intake manifold.  If the connector comes loose, or for any other reason the canister valve does not get energized, the intake manifold will suck air through the canister and the effect will be that of a large vacuum leak.  91 and later cars will compensate for the leak pretty effectively, but the engine will idle a bit fast and quite roughly.

Make sure the hoses are connected to the charcoal canister.  Remove the right inner fender well to allow access to the canister.  Remove the canister by pulling it up and out of its mount.  Disconnect the hose that goes to the solenoid valve (the other line, a vent from the gas tank, goes directly into the top of the canister).  Connect a length of hose to the valve and replace the canister in its mount.  With  the ignition switch ON, tentatively suck on the hose - no air should flow.  If it does, disconnect the electrical connector on the front of the valve and check for 12 volts with the ignition switch on.  If no voltage, check fuses and wiring.  If voltage, replace the valve.