I just changed all the spark plugs in my f-150 (horrible experience lol) and saw this in the engine and wasn't connected to anything. It looks like it comes from the drivers side Fuel Pressure on the rail? Is it supposed to connect into something?

 

I believe that's a vacuum line for a 4wd system. If your trucks 2WD they just cap off the line, i was thrown off too when I looked at a 2WD truck.