i replaced my expansion tank on my bmw 318i se E46 and filled her with water... i drove her out about 5 miles to a trade parts department and i had to stop 3 times cos the temperature guage went dangerously red... so i pulled over... unscrewed the cap, (carefully with an old che guevara t-shirt)..and filled her back up with water... anyway... i was told by the partsman... ii was told to bleed the radiator and then fill her up with 7 litres of coolant/anti freeze.. that's not what the RAC man said... he said, "after you change the expansion tank fill it up with water and let it run for a bit the unscrew the screw on the expansion tank hose"... so i did... but the coolant warning light comes on after about 20 minutes or so.... how much coolant do i actually need? and any other ideas to keep the warning light happy... the temperature guage is always sittin in the middle regardless of whether the coolant warning light is on or off... please??