you must be a wierdo.stupid.
Water pump defective.
If your radiator isnt leaking and car is having a over heating problem and coolant spewing out. Chances are your thermostat needs replaced. possible the radiator needs to be flushed. But more likley its thermostat
did you use the right anti-freeze or you may have a bad fan/fan switch
cooling system needs flushed out and a new thermostat installed, also if the radiator is still original it probably needs replaced or rodded out.
Could be your electric fan on the radiator is shot, or the relay is bad.
Have flushed complete cooling system replaced water pump radiator thermostat clutch fan added 2gal straight anti freeze then filled to capacity with water and truck is still over heating. What else could possibly be wrong
you might want to check for a blown head gasket or a cracked head or block. espically if it loosing water out of the radiator with out leaking on the ground.
Test gauge function with a mechanical gauge to get actual engine temp.
Low coolant? System air bound? Hoses collapsing under pressure? Water pump not circulating coolant?
System airbound? Coolant level? Temperature control cable?
It could be a clogged radiator or cracked head or bad head gasket.
The radiator on a 1999 Pontiac Grand Prix is flushed by draining the coolant, filling the system with water, adding a liquid flush to the system, and running it for a few minutes. The system is then drained, flushed with fresh water, and refilled with coolant.