03 hemi temp gauge rising, not heat

03 ram hemi temp gauge will rise as heat shuts off blowing cold air.....coolant is overflowing through overflow tube....whats causing this?

It's pressurizing somewhere.

thanks bleeddodge....they just changed thermostat and pressurized....give me a possible example if you could

Maybe the new thermostat was put in backwards? Did you replace it before or after this problem started?

Why would the thermostat need to be replaced on a 2003 truck? Did something else happen?

maybe backwards...i brought it back tonight...waiting for result tomorrw....nothing happend b4.....could be headgasket?...your thoughts?....thanks man

You could also have some air locked in/near the heater core. If the problem persists once you get it back, try parking with the front end uphill to let any air out; let the thermostat cycle a couple times and keep an eye on the radiator level. If there's air you'll probably see some big bubbles come out, and the radiator level go down.