Bad temp sensor or temp guage. Did you put the thermostat in backwards? If your head gasket is blown you won't always get white smoke out of the tailpipe. You can however get WAY too much pressure in the cooling system and blow a hose soon. Check the coolant for oil or any kind of dirt. Also go on www.freshalloy.com for any further 240 tech questions. They have more information about the 240sx than any other site.