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The thermostat housings on the 3 Series are a known area to develop cracks and leaks over time due to their exposure to thousands of heat cycles, which cause the plastic to weaken. This relatively inexpensive item should be on everyone’s list for preventative maintenance, or whenever replacing your thermostat. From Germany's Febi.
Consider replacing your thermostat at the same time, and make sure you have enough coolant on hand prior to starting the job.
Please note: Part numbers are provided for reference only. For correct compatibility to your vehicle, please follow our fitment advice