We are changing the water pump need to refill the system

When you changed these components and refilled the cooling system, did you purge the air out of the top of the system by opening the bleeder valve. These engines have coolant in areas higher up in the engine where air can be trapped during refill. Air trapped in the system will cause overheating issues. Otherwise, the only thing you haven't mentioned in the system is the radiator and fan assemblies. They could be suspect, as could a possible blown head gasket.

refill not bleed

3.3 AND3.8 ENGINES TAKE OFF SERPENTINE BELT 15MM ON BELT TENSIONER . REMOVE 2 HOSES OFF PUMP. 5 ?BOLTS AROUND WATER PUMP. IT WILL COME OFF WITH ANTIFREEZE LEAKING OUT. HAVE DRAIN PAN UNDER . ALSO IT IS GOOD TIME TO CHANGE THE LOWER HOSES. REINSTALL WITH NEW GASKET, FILL SYSTEM, PUT HEAT ON HOT AND RUN UNTIL THERMOSTATE OPENS, AND CHECK FOR LEAKS, TORQUE IF NESSESARY ( ENGINE OFF)!!REFILL AT SIDE TANK

Was the thermostat changed? I would suggest replacing the thermostat refill the system with new coolant and purge the air out of the system.

DRAIN COOLANT SYSTEM.DISCONNECT THE RADIATOR INLET HOSE FROM INLET PIPE. REMOVE THE TIMING BELT FROM THE WATER PUMP PULLEY. REMOVE THE RIGHT SIDE IGNITION COIL.REMOVE THE WATER INLET HOUSING AND THERMOSTAT.REMOVE THE MOUNTING BOLTS AND STUDS. LIFT OUT THE WATER PUMP BY CAREFULLY PRYING BETWEEN THE PUMP AND THE CYLINDER HEAD. REMOVE ALL THE OLD PACKING AND CLEAN THE MOUNTING SURFACES. INSTALL NEW SEAL PACKING TO THE WATER PUMP GROOVE AND A NEW O - RING TO THE WATER BYPASS PIPE.INSTALL THE TIMING BELT AND CONNECT THE INLET HOSE TO THE INLET PIPE.THEN REFILL THE COOLING SYSTEM. RUN THE ENGINE CHECK FOR LEAKS.

YOU HAVE TO REMOVE TIMING BELT AND IDLER PULLEY.YOU NEED TO REPLACE TIMING BELT WHEN YOU REPLACE WATER PUMP.TO REPLACE WATER PUMP. DRAIN COOLANT SYSTEM.REMOVE UPPER RADIATOR HOSE.REMOVE TIMING BELT AND IDLER PULLEY.UNBOLT AND REMOVE THE WATER PUMP.CLEAN ANY GASKET MATERIAL OR SEALANT RESIDUE FROM THE WATER PUMP MATING SEALING SURFACES.WHEN INSTALLING NEW WATER PUMP BESURE TO USE A SEALANT ON BOTH SIDES OF WATER PUMP GASKET TO KEEP IT FROM LEAKING COOLANT.TORQUE WATER PUMP BOLTS TO 13 FT LBS IN A TWO STEP CRISSCROSS SEQUENCE.INSTALL THE IDLER PULLEY.TIGHTEN THE MOUNTING BOLT TO 31 FT LBS.INSTALL AND TENSION THE TIMING BELT.INSTALL THE UPPER RADIATOR HOSE.REFILL BLEED THE COOLANT SYSTEM.

Check for the proper operation of the cooling sans.
Check the drive belt on the water pump.
You may need to reverse flush your raadiator.
Some cars require bleeding the air out of the system whrn you refill it with coolant.

Good Luck

5.3L Grand Prix Engine

1. Place a drain pan underneath the radiator. When the engine is cool, drain the cooling system from the radiator into the pan.
2. Remove the battery from the engine along with its tray.
3. Take the drive belt out of the engine.
4. Loosen and remove the three bolts to the water pump and remove the pump, throwing away the gasket.
5. Clean the gasket mating surfaces and install the new water pump with a new gasket, tightening the bolts to 89 inch pounds (10 Nm).
6. Reinstall the drive belt and the battery with the battery tray.
7. Refill the engine's cooling system with coolant. Start the car and check for leaks when the engine reaches normal operating temperature.

3.5L Grand Prix Engine

1. Partially drain the coolant from the cooling system.
   
2. Disconnect the surge tank outlet hose and remove the drive belt.
   
3. Remove the idler pulley and the water pump pulley, making sure to note the locations of the bolts as you remove them as they are various lengths.
   
4. Loosen and remove the bolts to the water pump, also taking note at their locations, and remove the pump.
   
5. Install the new water pump with a new gasket, tightening the bolts to 124 inch pounds (14 Nm).
   
6. Reattach the water pump pulley, tightening the bolts to 106 inch pounds (12 Nm).
   
7. Reinstall the idler pulley, tightening the bolts to 37 foot pounds (50 Nm) and install the surge tank hose and drive belt.
   
8. Refill the engine's cooling system with coolant.
   
9. Start the car and check for leaks when the engine reaches normal operating temperature.
   

3.4L and 3.1L Grand Prix Engines

1. Place a drain pan underneath the radiator. When the engine is cool, drain the cooling system from the radiator into the pan.
2. Disconnect the negative battery cable.
3. Remove the accessory drive belt guard and the accessory drive belt.
4. Loosen and remove the water pump pulley bolts and remove the water pump pulley.
5. Remove the water pump bolts and remove the water pump.
6. Install the new water pump along with a new gasket, tightening the bolts to 89 inch pounds (10 Nm).
7. Reinstall the water pump pulley and tighten the bolts to 18 foot pounds (25 Nm).
8. Reattach the accessory drive belt with the drive belt guard.
9. Refill the engine's cooling system with coolant.
10. Start the car and check for leaks when the engine reaches normal operating temperature.

If the water pump is leaking, then you need to replace it, replace the thermostat as well. Make sure you have got all the air out of the cooling system, when you refill the coolant, by undoing the top hose of the radiator till water comes out, likewise with a heater hose, then start and check, when the temperature is right the fans should cut in, keep an eye on the temp guage whilst doing this.
Hope thgis helps

Need to loosen bolts for the water pump pulley. Remove belt and then remove pulley. Should be 5 10mm bolts holding the pump on. Remove the bolts. Reinstall in reverse order. Refill with new coolant.