No need to jump to such terrible conclusions just yet. Sounds like you are familiar with the issues regarding this engine design and head casting.

The temperature rise is obviously dependent on load, as you have described. This implies higher fueling rates and greater cylinder pressures. These demand higher rates of cooling from your system. Any of the system's elements may be suspect, to a degree.

It is quite possible that some things may be failing that are easily fixed. (easy is a relative term, I know)

-thermostat (it can happen and cause this, but somewhat unlikely. However it is cheap and therefore worth the effort)

-Head gasket

-Obstructed radiator

-bad fan clutch (highly unlikely)


-and you are aware of the worst outcome, aren't you?

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