Ok i'm looking over the mustang i just got and there is a plug-in of some kind that is unhooked and i don't see any wires that would plug in to it. There are pics below of what i'm talking about, Oh just so you know its a 1991 2.3L hatchback or so im told.........Thanks for all your help
That looks to me like an engine coolant temp sensor, although it's in a completely different location on my '87. Is the casting it's screwed into part of the lower intake manifold?
Whether it's the ACT (air charge temp) sensor or the knock sensor, the car WILL run without it. The knock sensor protects the engine from predetonating (knocking/pinging) so it could possibly save your pistons if something major were to give out causing the engine to knock. Leaving the ACT unplugged may cause check engine codes to be thrown and might cause the engine computer to use a somewhat 'safer' fuel strategy thus possibly using slightly more fuel.
The knock sensor measures vibrations (knocks) from the engine and is usually screwed into the side of the block or the head. The ACT is always in the intake at some point.