Engine Coolant Reservoir Water Expansion Tank for 540i Coolant Radiator Reservoir Bottle with Cap 17111741167/17111723071 This coolant reservoir tank is designed to fit models requiring part numbers 17111741167 and 17111723071.It functions as an expansion tank within the cooling system, holding excess coolant and maintaining proper pressure levels.The unit includes a cap to seal the system and prevent leaks.Constructed to meet specifications, it integrates into the engine's cooling setup.Installation is straightforward, replacing the existing reservoir without modifications.This component helps regulate engine temperature by allowing coolant to expand and contract during.