Originally Posted by its cheddar baby
it is the crankcase breather tube, and if it is not connected you will indeed have a vacuum leak and a bad idle. If it is connected and just kinked, you'll have the vacuum problem but not the bad idle.
Technically, Ford calls it the Crankcase ventilation hose. The system is positively charged and has a PCV filter, which is why I refer to it as the PCV hose. When disconnected, you actually lose pressure but the Ford diag manuals call it a vacuum leak, go figure