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Hi Saverauto, it doesn't sound like those are US models, but this probably applies to those too, unless they're diesel motors.
They call it a two stage system, where the thermostat operates as a normal one, and the water control valve closes under 95*c to only let coolant flow through the head. Above 95*c and it opens and let's it flow through the block. The control is supposed to get the engine up to temp faster. The control valve has hoses going to the throttle body and the heater core. The thermostat is just from the block to the radiator.
Either stuck closed would cause an overheat. You'd know if the water valve was stuck closed if it is overheating, and the radiator inlet hose isn't hot. That would mean that the water is only going through the head, through a pipe off the valve, that would be hot, and it's not going through the block.
It is supposed to give better smoother control of runing temperture in a wide range extreme conditions. Longer ago than I care to remember when I was a young seagoing engineer it was used on small auxilary diesel engines ISTR Lister diesel but it might have been Petter.
A lot of manufacturers are now using weird and wonderful thermostat setups thesedays some of which work fine and some not so good.
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