coil packs.....and any drop in coolant level should be investigated. My coolant level doesn't move at all, even in the QLD summer so you can discount evaporation. I saw an R34 the other day that was dumping coolant on my driveway, the head-gasket was gone for sure even when the car wasn't displaying the traditional white smoke etc etc there should be no reason to have those sorts of pressures in the coolant system.