I've seen engines overheat without a thermostat. The tstat causes turbulence in the coolant flow which increases heat exchange. Without it an engine can experience a phenomena called laminar flow. The gist being that smooth flow current travels mostly in the center of a passageway and leaves the surrounding fluid almost completely stationary.
Sucks about the genset. At least you weren't on a paying job and it did that.
Sucks about the genset. At least you weren't on a paying job and it did that.