cooling system - small plastic black knob

[minkoil7]

C an someone tell me what this small black knob in the cooling system is. It is to the right of the radiator. I lost a lot of anti freeze and think it was leaking out from here. the knob was loose so I tightened it. What is it for and how tight should it be ? "Hand Tight"? 2010 Explorer AWD V6
Thanks,
-Jim

 

[Rcflyer330]

The red circle in the picture its the high side port for the ac system and it has nothing to do with the cooling system. It should be hand tight. Check in between the cylinder block where the thermostat is for coolant its a common place to leak from. Also if you have rear heat check the lines in the back under the driver's side. It could also be leaking from the radiator or a y connector near the passenger side of the car in the engine bay.