assuming you have a functional gauge .... what Scott said, if you don't have a thermostat the system can't work as designed - the rad is sized to drop the temp by 50* or more each pass, if you don't "force" the temp up with a proper rated thermostat, it'll never get there
get a good thermometer and check the water temp coming into the top of the rad to verify the gauge - after you get a thermostat
Bill
