Is my radiator pluged ?

I have a 89 Deville 142000 miles on a 4.5L . I don't know the history of this car. After driving 2-3 miles the Engine Collant/ Fan light comes on. At that point the upper radiator hose is real hot & rock hard, the filler cap is cool to the touch, if I open the filler cap about 1/8th of a turn hot coolant blast into the overflow tank. Do I start with the radiator first or flush the entire system first ? What about the thermostat ?
PS. The coolant fan is working. Any pointers? Thanks Dave

Sounds like the thermostat. Since the radiator cap is cool, that means that the water is not circulating. If it's still hot after replacing the thermostat, then start looking into the radiator and water pump. A stuck thermostat is the most common, least expensive and easiest to change.

Yup thats the thermo. But I say while you have your coolant drained out to change it flush the entire system and refill it with all new (unless youve just done that or you know it was done a shot time ago)
Once you drain it all out fill the entire system with water, then drain that. FIll it with water and drain it about 4 times or untill the its pure water draining out. Or keep doing it untill you see fit. Have the car running and the heat on high while you fill it with water.