Overheating concours deville 1999, changed thermostat, new coolant, no leaks, water pump seems fine

A worn water pump is the most likely. If the coolant is going down, the water is going somewhere.

The most common reasons an Oldsmobile Alero is overheating are a coolant leak (water pump, radiator, hose etc.), the radiator fan, or a failed thermostat. Coolant leak (water pump, radiator, hose etc.)

Oldsmobile Alero Overheating - RepairPal

Videos on overheating. You probably have air in the system from when you replaced the thermostat. The coolant system needs 'burping' of air.

https://www.google.com/search?q=2000+Oldsmobile+Alero+running+ho

Burping

https://www.google.com/search?q=burping+coolant+system

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Removal & Installation

1. Before servicing the vehicle, refer to the Precautions section.
2. Drain the coolant.
3. Remove or disconnect the following:
   
   Negative battery cable

Upper fill panel, if equipped
AIR injection control valve, if equipped
Air cleaner
Oil level indicator tube nut, if necessary
Water pump belt cover
Water pump drive beltLower radiator hose
Thermostat bypass hose from the coolant inlet housing
Water pump cover

Remove the water pump by rotating it clockwise using special Tool J 38816-1A (water pump remover/installer).
To install:
Install the water pump with a new O-ring, by turning it counterclockwise using the special tool, until it stops. Torque the water pump to 73 ft. lbs. (100 Nm).

Install or connect the following:

Water pump cover. Torque the bolts to 89 inch lbs. (10 Nm).
Lower radiator hose
Thermostat bypass hose to the coolant inlet housing
Water pump drive belt
Water pump belt cover
Oil level indicator tube nut, if necessary
Air cleaner
AIR injection control valve, if equipped
Upper fill panel, if equipped
Negative battery cable

• Refill and bleed the cooling system.

Run the engine and check for leaks.

Water pump vanes wear down with age. May need a new pump. Other things to look at are a cracked head. The engine will burn the coolant if there is a crack. With the engine cold. remove the radiator cap. Start the engine and let it run long enough for the thermostat to open. Now look at the coolant flow. Is there any bubbles in the coolant as it passes by ? If so then you have a crack in the head.

First, you must have a properly installed thermostat that is working correctly. Second, Northstar engines are notorious for air in the cooling system after draining and refilling - keep checking the purge tank level after overheating and after the cooling system cools down. Third, make sure your cooling fans are working correctly (relay, fuse, temperature sensor) - check to make sure both are running when it overheats. If all the above is correct, the problem will probably be blockage in the system (try a flush) or your water pump itself is failing.

change the coolant temperature sensor, if that's not it, look into the head gasket leaking.

These engines are know for leaking head gaskets. A good quick test for this is fill the coolant tank to the top with the cap off. Start the engine. If the the coolant blows out of the tank, you have a head gasket leak.

have you checked the waterpump it is 10yrs old could not be circulating the water, also have it pressure checked to see if it leaking or loosing pressure.

remove pump and check that the imèller which is plastic hasnt seperated from drive shaft

looked it up online and looks like it's on the front of the water pump in a thing called the coolant inlet housing. one end of it looks something like the letter y and the other end, where the thermostat is round. hope that helps

99 cadillac deville overheating put in new radiator,thermostat,waterpump. it stays cool at idle for a hour running.drivin about 15-30 minutes it overheats. is not using hardly any coolant to speak of.