[djomlas]

when my coolant cap gave out, it sounds exactly as you describe.
i was driving with my GF, and a bus driver said there is something green leaking out of your car, i pulled over, and all the coolant leaked out, and it leaked all over hot exhaust piping and cats, so there was a lot of white smoke.
filled it up with water, drove home, and on the way back kept refilling.
then did the coolant flush back home, along with a new coolant cap($12)..you will also need to purchase coolant.

do to the DIY section and look at my DIY for coolant, and check the coolant cap numbers and what coolant to use. in my case it wasn't the coolant tank.