1985 F150 / 351W / 4x4/ Manual

Wondering if anyone knows what the vent thing is called and what it does at the back of the engine. (See pictures)

 

That's a one-way valve for the thermactor air injection system that's rusted off. The top opening should have a nipple fitting for a hose that comes from the TAD/TAB control valves that are fed by the smog pump. That air is supposed to be pumped into the exhaust port when the TAD/TAB valves are configured properly by the EEC, and the one-way valve prevents exhaust gasses from pushing back to the control valves.

 

Melt wires and start engine fires. There should not be an "exhaust vent" in the engine bay, as mentioned the rest of this system has been removed by a PO or has otherwise left the mother ship, you whould remove the remaining crossover tube and plug the holes in the rear of the cylinder heads with 5/8x1" coarse thread bolts with copper washers to seal.