Yep, we had this problem with an inline thermostat in a Mitsubishi FGC20 some years back. That was the first time I had ever encountered an inline thermostat and at first glance I interpreted it to be a "check valve" due to the directional arrow on the cylindrical body. It is crucial that the inline thermostat be placed in the RETURN circuit hose so it can sense the coolant temperature LEAVING the regulator with the arrow on the thermostat pointing away from the regulator.