Cadillac deville 1997 keeps overheating already change water pump i dont know where the pressure valve is to get air out of the cooling system

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.
If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.
The hose goes in to the engine to a crossover and comes out the other side close to the thermostat housing,the blockage may be there.
If you still wish to replace the stat...
The thermostat in the northstar engine is located at the bottom radiator hose (unlike most conventional cooling systems), the hose connects into the housing where you will also find the water pump.

You will need to remove the air filter housing and air duct to gain easy access to the thermostat.

Remove the hose from the housing,then remove the two bolts on the flange,the thermostat can then be pulled out.

Take note of the orientation of the thermostat as you remove it.

The thermostat in the Northstar engine is a dual action device.
One end of the stem is the Thermostat(obviously) and at the other end is a spring loaded pressure valve(the thinner end).
Make sure the thermostat end is nearest the rubber hose and the pressure valve end is in towards the engine.

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.

Just keep the reservoir filled to the right line and the cooling system should take care of the air bubbles after a while, provided that the radiator cap is sealed for both pressure AND vacuum.


Performance Chip

REPLACE THE THERMOSTAT AND RADIATOR PRESSURE CAP.IF COOLANT LOW, ADD MORE COOLANT THROUGH COOLANT OVERFLOW UNTIL COOLANT LEVEL IN THE COOLANT OVERFLOW STAY AT FULL COLD MARK.REASON POUR COOLANT THROUGH COOLANT OVERFLOW MOST GM CARS DONT HAVE RADIATOR CAP.WHEN YOU GET COOLANT LEVEL OKAY, CRANK CAR LET IT IDLE UNTIL ENGINE GET HOT BUILD UP PRESSURE THEN TURN OFF CAR.CHECK FOR COOLANTS LEAKS AT THE RADIATOR HOSES,CHECK FOR LEAKS AT THE RADIATOR, CHECK FOR LEAKS AT WATER PUMP WEEP HOLE IF YOU SEE COOLANT LEAKING FROM WEEP HOLE REPLACE WATER PUMP. IF CAR OVERHEATING WHILE IDLING YOUR COOLING FANS NOT WORKING CHECK COOLING FANS FUSE AND RELAYS.IF FUSE AND RELAYS OKAY THE ENGINECOOLANT TEMPERATURE SENSOR BAD.

Do you have the Northstar engine? If so, there is an issue with head bolts pulling threads. Clues are periodic overheating. Look for exhaust in the cooling system. What happens is under load the compression gases push coolant out, and then you get overheat.
Pressure test the cooling system, there is a small hose that runs from the engine to the coolant tank, sometimes that plugs and air gets trapped. Some models suffer water pump impeller failure, easy to check with a special removal tool.
The head bolt issue is EXTREMELY expensive to fix correctly. Essentially you need to time-sert the bolt holes, and to do that you need to give a bare block to the machine shop.... Real bummer as the repair typically exceeds what is reasonable to invest in the vehicle, thereby rendering the vehicle useless.
I did repair mine, because I really liked the car.This issue should not happen.

There is much written about this issue...One small hose between engine and that plugs up. There is an issue with the head bolts and the aluminum block, Usually cost prohibitive to fix. There is a test to see if exhaust fumes are getting into the cooling system because of head bolts....

All car cooling systems have at least 10lb pressure, check with your manufacturer on your car's radiator pressure cap or local parts supplier. If pressure is alot more then I would suspect a leaking head gasket.

Easiest test for the obvious leaking head gasket is when the engineis cold / remove the radiator cap and fill right to the top with water and crank / start engine. If you see a mini "guyser" of water come out then head (S) need to come off.

You haven't mentioned whether its actually losing . using water??

Overheating can be numerous things: Fans not working, water pump, thermostat, overflow cap, and the list keeps going. When the engine is running and heats ups, feel the heat of the top hose, if the hose isn't hot, the thermostat is most likely stuck open and needs replaced. If it is hot we know you have circulation. The cap on the expansion tank may be defective, if it can't hold the pressure of the water, it will release at the overflow cap if the cap is bad. Also check the coolant level.

It sounds like they didn't get the cooling system filled completely, there's an air bubble. I had a problem with a 2000 Oldsmobile overheating, it turned out to be the aftermarket thermostat. I bought a stat from GM the problem went away.

head gasket is blown, creating pressure in the cooling system