i dunno about an air bypass valve.
there is a idle air control motor, that can be considered an air bypass valve.
it is common for TC to have high and unstable idle.
maybe this is what the ment. do you have a part number.
TC's did not have a bypass valve for over pressure in the intake from the factory, so maybe it is a mis communication in the name of the part.
Brian J Larkin
1988 Tbird TC 5spd. few bolt ons, 308k miles
1989 Cougar XR7 AOD 110k