may have imbalance in coolant mixture, may have bad water pump, may have bad thermostat. check each.
you have a bad head gasket
its coolant could be a head gasket or the coolant can actually rust trew the block or the head check the dip stick if its a creamy and it smells nasty or check you colant if it smell like cofe then you know coolant is getting in were its not supose to. white is coolant blue is oil and black is fuel hop that can help you
Remove Your Radiator Cap, If It Is Full OK. If Not Add Until It Is Full. Put Cap Back On. Now Fill Your Colant Reservoir. To The Cold Level. P>S> Be Sure Engine Is Cold When You Do This.
You can find it in the water outlet.
blocked colant passage gasket on wrong bad temp sen
tht intake manifold gaskets go bad on the 3.1! they have revised gakets to take care of problem, there has been an issue with the 3.1 gaskets for sometime and GM doesnt care! the dex-cool colant seems to eat up the gaskets.
Low Colant level, Bad thermostat, Bad sensor, cracked head
You would have to get a repair manual like a Haynes/Chilton.
Could be your water pump leaking from the gasket or,, under the reservoir where the over flow coolant is,, check all the small hose coming in and out of it also the one that go to your transmission,,,leave a cardboard paper under car and see if it drips,,goodluck.check your cooland level
when it does not start disconect your cts (colant temperature sensor )and then start if it starts replace your cts
Do a search for your ects sensor. I am trying to find out how many temp gauge sensors are on the car right now myself. I am having the same problem. I am going to change the temp gauge sensor(s) and thermostat and flush the radiator.
Check the colant level, it may be low.