overheating problem

i have a pontiac grand prix (1994) and have been having problems with overheating. i have about 3 minutes before the temp shoots up to 260. i tried removing the thermostat to see if it would create an immediate result however it made no difference. my coolant tank is full and there is no sign of leakage from anywhere. could this mean its the water pump? how easy would it be to replace myself? i cant afford to spend very much, so if its feasible i would like to be able to replace it on my own however i want to know if it could be another problem before i start taking everything apart.:banghead:

When it shoots up high do you see any steam anywhere?

What motor is it? why dont you put a new AC delco thermostat in it, and then make sure you bleed all the air from the cooling system.

well the problem no longer seems to be the thermostat. and no, no steam anywhere...

Let's ask again, what engine does your car have?

While we're at it, how many miles, and is the CEL on?

good point Rich.... Are you basing your overheading problem just on the guage or does the motor get very warm? Next time it is started why dont you feel the upper and lower radiator hoses just before it overheats. Note the pressure and also relative temperature. Dont burn yourself. Something tells me your going to have a very high pressure right after startup.

Sounds to me like you have either another bad thermostat, air not purged from the system when you changed out the old one, not enough coolant in the system, water pump impeller spinning on shaft, collapsed hose, or heaven forbid, a bad head gasket.