sounds like too much grill block! the old kia did the same! you use the engine too much to ignore it tho. it wasn't a big deal for the kia because I only ran the engine for 30 seconds at a time and turned it off before it could realize it was overheating and knocking / misfiring.
the kia is gone now don't be a fool like I was...
( mpg is high at 237*F)
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