Chris, did you carefully inspect all the spark plugs? If the #5 cylinder is
the only one running really hot, the plug should look distinctively
different from the others. I agree with Dick T. that if the problem is
isolated to the one jug, that it is likely to be a vacuum leak into the
intake port of that individual cylinder (warped manifold, etc.), or
something like an exhaust valve not completely closing. General overheating
can be many other things including retarded ignition timing, late valve
timing or, lean mixture. A fellow British car buff friend of mine has a
Suburban as one of his family vehicles that would get both exhaust manifolds
glowing cherry red after a good long run. The cause turned out to be a
combination of inadequate spark advance at speed coupled with a stretched
timing chain which retarded the valve timing. If the other solutions don't
pan out, you should be able to re-verify your valve timing without removing
anything more than the valve cover using a degree wheel and a dial
indicator.
Regards,
Rex Townsend
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