Today i noticed the coolant res.tank was empty on coolant so i filled it back half way and then when i got home i filled it almost to the max line with drinking water. After driving aliilte bit, i check the resivoir(sp) tank and the damn thing was all the way to the top! Will the have an effect on the car or the coolant system in any way?

Thank you.

If it's significantly overfull, it will find its way out somewhere when it expands- this is usually from the cap (This usually happens the first time an engine warms to full temp after filling) but can also be from another weak link- like an old original radiator that's on it's way out. If it's really wAy too full, you would be better off draining a little out to get it to the right level.
DO NOT open the cap when hot or under pressure. I personally know someone who was BADLY burned by this.... still has the scars 15 years later....
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Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K