Where does this hose/line go?
 

Found this hose/line while doing spark plugs, on the top of motor, passenger side in the back and runs back and down by the firewall. Was kinda hanging next to #3 cylinder. Looks like a vacuum line that came disconnected. One side was dirty and the other (what I would call the inside) was very clean. Doesnt look broken....more like popped off. Does anyone know what its for/where it goes? Truck is a 2004, F150, 5.4L, 2WD and runs fine. Idle does seem to be off just a little now tho.... Thx!!
https://cimg1.ibsrv.net/gimg/www.f15...ae9ea3a630.jpg
https://cimg5.ibsrv.net/gimg/www.f15...edaca9630b.jpg

I believe that's a cap for the iwe/4x4

Without 4x4 on my truck, does it need to be plugged in/connected somewhere? Seems odd that it was so clean on one side. Couldnt see any connection point where it would go tho.

That line is intentionally 'killed', 'plugged'. It is sourced from the very bottom (belly) of the Intake Manifold (for stable vacuum source) to supply the vacuum reference to a bank 1 Fuel Rail Pressure sensor - which is not used on your truck. The vacuum source connector has TWO lines coming out of it - the other one goes to the vacuum reference port on the Bank 2 (common) FRP sensor. Just leave it lay. For sure don't pull it off or dislodge it from its vacuum source connector at the bottom of the intake manifold. You'd have to remove the intake manifold to reconnect it. That is a bitch!

Awesome! Thanks a bunch for the detailed reply. Makes me feel better knowing that it isnt a rabbit hole I need to go down before a big trip I have coming up. THANK YOU!!!

Damn thing worried me sick on my '04 5.4L 3v until I had to remove the I/M and figured out what the deal was. I had taped it up and put silicon seal on it trying to figure out if it was the source of my vacuum leak (would up being a crack in the plastic IM). Ugh!

Thanks torque.