can you post some photos.
that's a strange problem indeed.
can you tell if it's stuck wide open, or just a little open?
as the other guys say, this could be the cold idle cam/choke (if equipped) set incorrectly causing this.

tried carb cleaner and lube?

and if recently worked on, sometimes the throttle shaft nut if it's too tight can cause binding.

msg - '87 xtracab