Part #12 in this diagram:

(From slhondaparts.com, for 2000 MT Base Lude, engine, throttle body page)

Its on all 4g/5g preludes. It is supposed to be sucking air, until the car is warmed up. Then once the thermostat opens, the hot coolant heats a wax plug, and closes it off. The wax plug deforms/fails with high frequency. You have 2 choices if you don't want to replace it...do the FIV/IACV coolant bypass modification (if you live anywhere its cold for extended periods of time, or strict Air Restrictions Board don't do this...) or cut an aluminum can/sheet to insert between the FIV and the TB.