A hydrocarbon trap assembly for use in an intake system of a motor vehicle engine includes a first duct having an interior recess adjacent an end thereof, the recess having a blind end; a second duct overlapping the first duct's end and closing off a second end of the recess; and a thermoformed hydrocarbon-adsorbing sleeve inside the recess and having a first edge adjacent the blind end and a second edge adjacent the second end of the recess. One or both ends of the sleeve may engage a hook formed on the respective first and/or second ducts. The second end of the sleeve may have a flange that fits into a gap between the ducts.