if it's coming out the exhaust manifold it could be a blown head gasket or cracked head.
it could possibly have either a blown head gasket or intake manifold gaskets leaking
water pump. thermostat.
a new heater core
On the 3.1 LS model, the thermostat is in the intake manifold, just underneath the air intake, and between the exhaust crossover and the manifold. Trace the upper radiator hose back to a metal pipe from the engine, that is the thermostat housing. Replacing it means using a ratcheting box-end (10mm?) or open end wrench to start and finish, and fingertips for the rest, so NOT when the engine is hot! The thermostat fits horizontally into the manifold, and comes with its own molded gasket, which fits into a recess, and doesn't require gasket sealant. I just changed mine, so good luck.
check your intake manifold gasket.
the thermostat is located behind the glove box retained by two Phillips type screws err no that's the resistor pack for the blower motor,the thermostat is located below the inlet manifold with a plastic top attached to a steel pipe running the length of the block and down to the rad,three t8 bolls hold it on and with patience can be changed without manifold removal ,the steel pipe does however need undoing
If it is leaking coolant. And you changed the thermostat. I would then move to this section. Flush the radiator and the heater core. This can be done at a certified shop with a reliable mechanic.
have you changed the battery in the thermostat?
Sounds like thermostat is not opening and needs replacing
Check the Blender door... Also check the temp knobs they make be broken.
How did you change the thermostat?
that's what it is, i have a lumina, had same problem, replaced the intake manifold. run like a top since