No one can explain it better than you can gilly, you should become a teacher for automotive or mercedes benz hahah.. so that explains the white stuff in the coolant reservoir.. it looks a bit like carbon, in my opinion.. the coolant doesn't have to be changed that frequently.. I've seen lots of cars at my shop who never changed the coolant in the last ten years.. maybe they would replace a radiator.. or thermostat.. or even the heater block occasionally.. I know its preventive maintenance.. I remember you mentioned every 3 years coolant should be changed.. do u agree with this? from your instructions that means i don't have to remove the thermostat or open the air bleed valve.. this sounds very easy! haha thanks for your quick response...