coolant dumps from expansion resivior discharge tube 99 pontiac grand am se 6 cyl?

[Mark]

I suspect the thermostat is stuck, The o-ring on the old cap was bad, I bought a new cap, but the problem still happens as soon as the car reaches pre boiling temp. what would be the cause of this? iand the most logical path of trouble shooting.

 

[melsg5]

Replace the thermostat and flush the cooling system

 

[Aaron123]

symptoms of bad head gaskets

in your case, the problem u have is more than likely engine head gaskets are bad. try ez way to check. first, remove the cap from coolant overflow tank,and start the engine and use the hand to feel any air come from the tank, or smell any burned smell or not.
that problem is very common on GM 3.4, 3.1L OHV ENGINE which your car has. because when engine running, the compression air from cylinders will go through the bad gaskets into coolant system that will build up too much pressure in coolant system even when engine is cold. that why your car had coolant leak from there.
change the head gaskets and a thermostat. hope it helps.