What kind of headers are they (I'm assuming aftermarket) and what are you putting them on?

Prior to 1986 the 5.0l Mustans' had a heat riser attached to the driver side exhaust manifold. In order to retain a functional emisions system there are some people out there who took their aftermarket headers/exhaust and "modified" them to accept the heat riser. Then again, some people such as myself just yanked all that shit out of there and put a '86+ style exhaust w/o the riser.

Open the hood and take a look at the driver side header. Follow it down the exhaust path to were the collector would be. If you see an odd looking metal gimmick with a vaccuum hose attached to it...then that would be the heat riser.