[srbianats]

check the level in the radiator and barf tank. also, your system might need to be burped. you might have air caught in the heater core. park your blazer at a incline where the front end is higher that the rear. run engine to operating temp with the radiator cap off. fill if needed. fill the barf tank to level and start to rev your engine a little to force the air in the core out. hopefully this helps. I had to burp my system after replacing gaskets. also had to burb again after replacing heater core..