I just installed a new Auto Meter water temp guage and sender in my 69. The guage goes past 250 after about 1 minute after start up if cold but the car does not overheat. Could there be a problem with my install? I also replace my thermostat.
If it's a minute of idling I'd say it's the gauge install; that's too much of a temp increase for such a short time. Did the old gauge work prior to the new one being installed? Check the leads and ensure you have the correct sender.