Cooling system

I have a problem with my cooling system, every time I drive my car then, I park it I i open the expansion tank an the cooling is still cold all I here from the expansion tank is like gas coming out for 10-30 second then I can completely take off the expansion tank. And my cooling light stay on it's my water pump not working anymore !?!?! I just change the the thermostat and expansion tank because my expansion tank exploded and my was heating it was over 100c now that I replace the thermostat it stay under 100c, I'm concern about the expansion tank cooling not been hot like it used to.. And my lower hose it's gets hot just like the upper one I though the lower one should stay cold.. Any tip pls help





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