Engine vent

[C Holmes]

Hi, was cleaning the frame on my 87 and noticed a flap (door) on the drivers side in the front. Seems to be spring loaded and quite stiff to move. I imagine it must be for cooling on a when needed basis. I assume then there should be an engine tray enclosing the bottom or what would be the point. Anyway, what makes it open and close? I can't see vacuum line or where one should be, no wires and it seems too stiff of a spring to open from engine heat alone. Any idea?