chances are you don't have it full enough and there is a bleed valve by water pump
Not enough air movement through the radiator, or around the motor; not enough radiator surface for the size of the motor / no thermostat, too much water movement through the radiator (water not cooling in radiator). - (Ignoring other obvious problems).
If you have replaced the water pump then flush the radiator with CLR also check to see if the fan is working properly.
You most likely have a blown headgasket.
The radiator on a 98 Buick Century is flushed by turning the petcock to drain the coolant. Water is then flushed through the system until all contaminants are removed.
There might be a radiator pipe running through the slab at that point.
Is cooling fan working? Radiator clogged? Air flow through radiator resrticted? Temp gauge accurate - check with manual?
Try checking Water pump, Radiator, cooling fan that is in front of the Radiator. With the Car running pop open the Hood. If the fan is not moving , it should be serviced. Also at that time the water pump should be replaced. Try bleeding the cooling system. they is a screw on the right side where the radiator hose is connected to the engine. Loosen screw and run water through the engine and make sure ALL air is removed. Keep checking temp guage. Hope this works , it did for me.
A hole in the radiator would do that, or a damaged radiator cap. It could be a cracked engine block that is allowing cylinder pressure into the cooling system, pressurizing the water until it breaks through a weak spot in the radiator.
coolant only flows through top hose when engine is at normal operating temp.. so u have the wrong thermostat, or no coolant in the rad.... Or the water pump has failed and is not pumping the water.
It could be transmission fluid. Many transmissions are cooled by running a line through the radiator. Check for small tubes going into the radiator, if you only have the two large engine coolant lines, you don't have a transmission cooling line in the radiator.
The radiator fan is designed to run continuously while the engine is running so that the temperature of the coolant can be lowered. The coolant is continuously run through the engine, collecting engine heat and dispersing it over the radiator fins; while the fan blows the fins and cooling off the coolant before recirculating back through the engine in a continuous cycle.
if your engine is running hot and its not your radiator you probably need to burp it to get the air out from the hoses and radiator, that should take care of the overheating problem, if it doesnt pressure test your hoses and raiator you might have a collant leak creating hot spots