[MagicRat]

This is the passenger side manifold, right?

I think you mean the heat riser valve. This valve can be removed. Ideally it helps a bit with cold weather starts and faster warm up, but they often malfunction and get stuck closed which costs you power and fuel economy.

I have wired the valve open on my GMC 2500 van with good performance results, yet it still runs fine in the coldest Canadian winter.

As for the hot manifold.........this valve should not cause that. I think you may have a burned or leaking exhaust valve or two on one side, or poor combustion due to bad wires, plugs etc on that side.