Mystery hose

[gtyates]

I was looking around the engine compartment of my new to me 2009 Sport Trac 4x4 and I have noticed a hose that is a bit of mystery to me. The hose is under the battery trey, sitting on top of the wheel well. The hose is black and has a white plastic cap on it that has a round end on it. What is this hose? Is this a vent for the front differential? Any idea?

 

[SMSgt09]

STA seen the same hose on my 09 STA 4.6. I saw it just laying there and thought OK if it ain't broke don't fix it. I think if is a vent as well although I wasn't curious .enough to try and figure out what it was for.