Bought a 1990 Mustang GT, and it has had the smog pump taken off of it.  The EGR valve isn't hooked up, and there is a vaccumm hose coming off the upper intake to the right of the throttle position sensor ( Cobra upper intake) that they had hooked to the fuel tank vent.  The hose had a hole burned in it from the header, and I fixed the hose, and know it idles real erradic.  I can unhook the hose and it runs fine.  If i plug the hose off it will die.  It has a mild cam in it. Don't know what size, but has a lope to it when it idles.  does anyone know where this hose goes.thanks  

60 views and no reply.  I thought you guys were Mustang guru's!!!!!!!!!!!!!!!!!!!!!  

the only hose i can think of is the coolant bypass hoses of ethier side of the throttle body.  Try tying them together.  if not then it has something to do with the smog system.  if it runs normal with it unplugged then leave it that way.