Had the car for 3 yrs. no problem. Two days ago (30 degrees) started and idled for 15 minutes. Gauge went almost to hot, heater blew cold air. Replaced thermostat, refilled with fresh anti-freeze...same problem. Felt thermostat housing and it stayed cold. What's next? Can tolerate cold air but worried about overheating.

 

Feel the hoses to the heater core. Both should be hot. If one is hot and the other is cold, then the heater core is plugged.

You can try to unplug it by carefully disconnecting the heater hoses from the core and using a garden hose reverse flush the heater core. Or simply reverse the heater hoses, I found that using Prestone SuperFlush works well. Just follow the directions.

I'd check the radiator for scaling and missing fins.

You may not have gotten all the air out when the thermostat was replaced. Trapped air could be preventing the hot coolant from getting to thermostat and then the radiator. Is the top radiator hose hot?

 

Thanks for the response...the radiator was new when I got the car. It and the thermostat housing were both cool to the touch while the gauge was showing hot. However steam came out of the reservoir when I opened the cap. I'll check all the rest of that in the next couple days and post what I find. Again, thanks.

 

I ditched the thermostat on my 02 fifteen years ago - takes a few minutes longer to warm up but I NEVER have to worry about blowing the engine due to a stuck thermo.

 

Any problems if I bypass heater coil by connecting in and out hoses on 99 Cavalier? Getting warmer out, can do w/out heat in cab till I decide what to do with coil.

 

Do it, save yourself a lot of aggravation. I live in a warm place, so the only thing I use the heater for is cooling the engine when it starts to get hot in traffic - heater on/fan on will pull heat out of the engine.
Don't hesitate to ditch the thermo also if you continue to have problems.