yes, as far as I understand it, the leaking exhaust manifold can cause the combustion chamber to get lean, so it got hot enough melted the spark plug. Also I heard a while ago that open headers or no exhaust manifold/headers at all can even cause the cylinder head valve(s) to get bent when the engine turned off. I am not sure if that's true but theorically it make sense due to cooling too quick.
Its possible for an exhaust leak to cause a lean condition. If the overlap on the cam holds the exhaust valve open long enough that the exhaust valve is still partly open as the piston starts the intake stroke it will suck in cold/fresh air from the leak. Problem is, it typically would have to be a significant leak to cause damage.
Also I heard a while ago that open headers or no exhaust manifold/headers at all can even cause the cylinder head valve(s) to get bent when the engine turned off. I am not sure if that's true but theorically it make sense due to cooling too quick.
I have never heard of it bending valves, but have heard of it burning (melting) the exhaust valve. This can only happen on an engine run with out any manifolds or with extreemly short open headers. If the header pipe is at least 10" to 12" the tubing will radiate enough heat to raise the tempature of the outside air to the point it will not cause a problem.
EDIT: BTW for anyone interested, that is a plug from a 5.4L 3 valve engine. Worse spark plug design ever.
Which side of the engine? I've replace a dozen or more passenger side manifolds on 3 valve 5.4's, drivers don't seen to leak as bad and yes a exhaust leak can cause that.
Yup, on the 3-valve modulars, that's pretty normal just pulling the plugs.
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