I have a 99 chevy blazer with 4,3 ltr v6. It was overheating so i replaced the thermostat. that worked for a cou[le days. now its not overheating it is spraying antifreeze out of a blowhole under the radiator after i shut it off. any ideas . is it the temp sensor?

 

Have you replaced the radiator cap?

Also, have you made sure that you are getting all of the air out of the system? Do so by getting the front of the vehicle as high as reasonably possible with the passenger side slightly higher than the driver side. Then run the engine up to operating temperature with the radiator cap off. Let it cool and top off the radiator level. Repeat 2-3 times until the level goes down consistently each time it cools off. Also, while it is warming up, watch for bubbles in the radiator that may be signs of a bad head gasket.