My 1998 chevy lumina is over heating. has a new thermostat and fresh coolant

engines overheat with no thermostats. Very simply put the water circulation is too fast for the heat to transfer to the air. Water pumps fail from bad bearings and pump seal but for no other reason. So back to basics Refit with a new thermostat put in properly. Have a flow test done on the radiator for blocked cores. have a coolant system pressure test done for leaks and cap failure.. have a compression test done to check for head gasket or cracks.. Check for correct ignition timing and air/ fuel ratio mixture. Run fault codes . when refilling with coolant ensure that all air locks are removed

Has this happened suddenly? Or over a period of time?
Either way, it sounds from your description as if either the fan/s that assist in forcing air through the vehicle's radiator aren't operating; or, as likely, the radiator core is partially blocked with sludge left over from when the engine was manufactured. Might be worthwhile getting someone to do a cooling system pressure check.
Good luck!

With the engine cold remove the radiator cap. Start the engine. Watch the coolant. If you see bubbles constantly perkulating out of the coolant than you most likely have a head gasket that has failed. If you don't see a constant perkulation of air bubbles it probably wouldn't hurt to replace the upper hose and see if the problem is resolved.

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On the 4 cylinder it's the bottem one on the thermostat housing
On the 6 cylinder it's adjacent to the egr pipe.

Not Familiar with vehicle but can give you some basic info.The Thermostat could be on top or bottom of engine.It is in a bell shaped housing,follow the Hoses from the Radiator.If you get heat when the Engine shows warm and you don't overheat it's not the Thermostat.If your not getting heat check the level of coolant first.If coolant is low you wont get heat or low heat.If your overheating it could be other problems,Blown Head Gasket.Clogged Radiator,insufficient coolant.Good Luck.

Sounds like a bad thermostat. I would start by replacing the thermostat. Also, make sure you have the correct mixture of antifreeze to water.

make sure all the air is out of the coolant system.

FIRST OF ALL YOU HAVE TO DRAIN SOME COOLANT.JUST ENOUGH SO COOLANT WONT FLOW ALL OVER ENGINE.AND COOLANT TO EXPENSIVE ON THE GROUND.TO GET TO THERMOSTAT.YOU HAVE TO DISCONNECT CONNECTORS TO MAF EGR IDLE SPEED CONTROL AND IAT SENSOR.. REMOVE THROTTLE AND SHIFT CABLES FROM BRACKET.REMOVE AIR FILTER ASSEMBLY WITH MASS AIR FLOW SENSOR.REMOVE BOTTOM PORTION OF AIR FILTER HOUSING.REMOVE COOLANT HOSES FROM THROTTLE BODY.THEN REMOVE THROTTLE BODY ASSEMBLY.YOU HAVE TO DISCONNECT SMALL COOLANT HOSE TOP WATER PUMP.REMOVE SMALL BOLT THAT HOLDS METAL COOLANT LINE TO WATER PUMP.THIS LINE HAS TO BE MOVE JUST ENOUGH SO YOU CAN REMOVE THROTTLE BODY.ONCE THROTTLE BODY OUT THE WAY YOU CAN REMOVE THERMOSTAT.BESURE WHEN YOU PUT EVERYTHING BACK TOGETHER.REPLACE THROTTLE GASKET AND THE COOLANT HOSES COME OFF THROTTLE BODY BESURE TO USE OLD COOLANT HOSES MEASUREMENTS FOR NEW HOSES MEASUREMENTS MAKE SURE YOU BUY THE CURVE HOSE FOR THROTTLE BODY. DONT TRY TO USE STRAIGHT HOSE IT WILL KINK AND CAUSE NO COOLANT FLOW.WHEN YOU ARE DONE.FILL COOLANT BACK TO CORRECT LEVEL AND BLEED COOLANT SYSTEM.

You need to check your coolant level first. If it is low you have a coolant leak. No hot coolant=No heat

Sounds like just a vacume line to me...