You may also have to check if there is leaking problem with your engine cooling system (expansion tank, etc...). If no leaking problem, just ask your mechanic to fill up coolant or you can DIY, distilled water is ok. BMW cars have excellent cooling system, it won't boil over if your cooling system has no leaking problem.

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--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.