[liona]

Greetings! I'm a newcomer in this forum.
I've had this supra for over six years without any major problems, until last week when the engine became very hot (almost approaching the red mark if I did not stop to cool down), even with all fans working ok.
The following day, I filled the radiator full and the coolant reservoir to proper level , drove five miles and the temperature shot back up to approaching red mark again. When I opened the hood, the reservoir was overflowing with very hot bubbling coolant.
I am definitely losing coolant, almost emptying the radiator. I do'nt see any obvious line leak or hose break. In checking the oil dipstick, it did not seem to have any "watery or bubbly" coolant like substance or anything other than engine oil (my oil have just been changed recently, so it was still clear). I also thought that the level on the dipstick should have substantially gone up, if the coolant got into the oil chamber considering the large volume of coolant lost.
Could it be problem with water pump, thermostat or whatever other malfunction causing engine to overheat. The coolant appears to be flowing to the radiator well, but I am not sure if it is coming or circulating well back to the engine. How do I check if the water pump is bad?
I only know very little about engine stuff. I would appreciate very much any help or advice. I've only got 91,000 actual mileage, as I only have a short home to work commute. THANKS MUCH IN ADVANCE FOR YOUR HELP!