I have a 2003 Honda Civic that I've been having overheating issues recently changed out the thermostat and the radiator there's no signs of a head gasket a bad head gasket but it can be cold and not even being driven but as soon as I start the car and take out the radiator cap it explodes with water after it's just been sitting I'm trying to figure out what's going on with it I'm at a loss

 

Sounds like a HG. Exhaust is blowing through the gasket into the water jacket causing a sharp but immediate spike in coolant pressure. What have you done to rule out a headgasket problem?