The Peugeot Boxer 2.2D-3.0D New Engine Radiator Coolant Overflow Bottle Tank Reservoir (Part Number 17137539267), also compatible with select BMW models, is a critical component designed to manage coolant expansion and maintain system pressure in diesel engines. Used in Peugeot Boxer vans (2014 onward) equipped with 2.2L or 3.0L diesel engines, as well as certain BMW commercial or light-duty vehicles (e.g., shared-platform models), it stores excess coolant during engine heat cycles, preventing overflow and pressure surges. When temperatures drop, coolant is returned to the system to maintain optimal levels and minimize air pockets. Constructed from high-density, heat-resistant plastic, it features a translucent body for easy visual monitoring of fluid levels and a pressure-rated cap (typically 1.4–1.5 bar) to ensure system integrity. With a capacity of approximately 2–3 liters, it integrates seamlessly into the engine bay via OEM-specific mounting brackets and hose connections. This reservoir reduces coolant evaporation, prevents overheating, and protects critical components like the radiator, water pump, and turbocharger. Its corrosion-resistant design and reinforced fittings ensure durability in high-mileage commercial or fleet applications.