I have been having cooling system problems with this 2000 blazer w/4.3L V6 for a year now.
The newest problem is overheating within the first five minutes of driving. I pull over and give it a few minutes, then open the radiator cap(spewing out approx. 1 gallon of water), refill the radiator with what i believe is replacing what was spewed. Then the car goes to normal temp and has no more issues. Of course this repeats when i come home. The overflow tank fills up when this happens.

I do have secondary symptoms. The heater does not get extremely warm til after overheating. I pull over as soon as it starts blowing cold air, and then after i get moving again, the heat is nice and hot, and the temp gauge reads just below 210 degrees, and stays there. I replaced the thermostat for the second time last week, so i am sure it is not the issue. The water pump is exactly a year old, radiator is 3 months old, and heater core is a week old. All hoses have been replaced within this year period.

I have put a pressure tester on the system and I am holding pressure, so i know its not a head gasket.

I hope there is an expert out there that knows something that I don't

 

After trying a new radiator cap, I decided to drive it to work again. No joy, but when i got to work this morning, i found the return hose on the top of the radiator collapsed. I will replace that this weekend and try again. Wish me luck

 

Are you sure you've got rid of any air pockets when you refill the radiator? The larger-diameter hose on the driver's side of the radiator (top) is the one that usually gives me trouble.

 

Yes, I bleed out the air everytime i break the system open.

 

Replaced the hose this morning and went for another drive. The temp gauge headed for the red, and the heat quit again. Pulled over and noticed the overflow tank was full again, and a trail of coolant was behind the car. So no joy on the hose. I topped it off, and drove it around for 45 minutes, with no more issues. It seems that it overpressurizes until the thermostat opens, then operates normally. Heads and Head Gaskets are the direction I'm heading, unless someone else has a clue.

 

I'm having the same problem, except mine is not overheating, it when I turn off the engine, a large amount of coolant goes into my overflow which bubbles and steams. I too have replaced the LIM, radiator,and thermostat. Over the winter I hosed out the heater core because my heater wasn't working. I am clueless as to what is wrong