Overheating, heater blowing cold

sounds like the headgasket has gone, your automobile has had it.

Did you purge the cooling system after refilling it? It's pretty hard to blow a Chevy head gasket. Check this out on how to purge the cooling system... www.youtube.com/watch?v=xeTgmdRkQtY

so to get me on the same page as you. bad thermostat. you replaced it, now it over heats?

since gearheadRHINO said head gasket, most likely wrong, check your oil color if its anything other then black or yellow (yellow only if its new that is) get back to me i'll be happy to help

I’m having the same issue. Oil is not milky or different color. No heat inside, I think the fluid froze inside the line somewhere. But running into radiator and building pressure. Coolant is warm to the touch when engine is getting close to overheated. I would think the heat from the engine should defrost/ thaw the frozen area. It is -24° today though with a windchill of -50° so I may just have to wait and see. If it fixes itself in warm weather I will post. That may be until two or more weeks though.

I'm having same issue in 2004 Tahoe. Heater blows cold air and overheating. When it does blow something other than cold air its Luke warm. Oil is normal color. Its above freezing here today so I doubt its frozen fluid.

For me, my heater core had frozen. I put a 15$ space heater in and ran an extension cord to the truck, then turned the key in and ran the recirculated air until it felt as warm coming out as it thought it should, ran the truck and it was good. To be fair I had a small leak in the lower radiator hose that went unnoticed as there was a lot of melting snow and ice consistently.