Originally posted by jack_of_trades
I doubt this is your issue but a leaking waterpump o-ring can cause your coolant to be forced out of your overflow tank too. When the pump is running it can suck in air and force the coolant out. When the waterpump isn't running (ie: car is off) you'd have coolant coming out of the leak if this was indeed the issue. Like I said, doubt its your problem but I figured I'd just throw this out there for anyone else searching who may have a similar symptom.

You might rasie a valid pint but it is unlikely that what you explained is the problem. It is not nessary for coolent to be leaking from the engine to know that it is a blown headgasket. It may not be(a blown headgasket); however, I am quite sure that this is the problem. When there is boost pressure there is an effective increase in compression inside the cylinder thus forcing the combustion chamber gases into the coolent passages and then forcing coolent out into the overflow. I know this is happening becasue when I boost I can hear the belt squeeling indicating that the coolent is overflowing onto the belt. And just so someone doesnt say that mabye a belt is loose they are properly tensioned and did not squeel before this problem. Also at idle it is evident that there is air comming into the coolent overflow.