No coolant Thermostat not opening Defective temperature sensor
Low coolant Water pump not working properly Cooling fan not working Thermostat not opening fully Radiator plugged or dirty Check the radiator fan.
small amounts of water coming from the muffler is normal but if its antifreeze its not
The thermostat is probably stuck in the open position.
The primary cause of a thermostat malfunction is corrosion. It prevents the thermostat from opening and closing as it normally would.
Follow the top radiator hose, to where it intersects with the motor. This would be the thermostat housing. Remove this, and the thermostat will be exposed.
The gas tank in my extended 2000 Pontiac Montana is 25 gallons. I would imagine the 02 is the same.
A common problem with the Montana is the power steering pump.
Insufficient amount of coolant or thermostat not opening
Two symptoms of faulty thermostat which would more than likely require replacement of same: (1) engine overheats quickly (2) engine takes too long or doesn't reach normal operating temperature
coolant levels or the thermostat
One thing that would cause your 1999 Pontiac Montana to stall at idle is the idle being set too low. You should raise it to see if that fixes the problem.