Coolant in the top tank at temp is what is passing the t-stat, should be the operating range of the unit, if the gauge is showing 200 and the top tank is at 190 to 170 then the gauge is probably off, cannot get that wide a degree difference that fast, thermometer need to be in the coolant stream. If it is 200 then you need to pull the stat. Tube type thermostats if this one has one require a seal to keep from bypassing to much coolant, if it has a tubular style replace the seal gently.