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Help CenterBMW E60/E90 Expansion Tank Cap ，BMW Coolant Reservoir Cap
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The expansion tank cap on BMW E60/E90 models is tiny, yet it acts as the cooling system’s “pressure manager.” Its core jobs boil down to three points:
It builds and holds a pressure of about 1.0–1.4 bar, raising the coolant’s boiling point from 100 °C to roughly 120 °C. This boosts heat-transfer efficiency and cuts evaporation losses.
When system pressure exceeds the rated value, the pressure valve opens automatically and vents excess steam/coolant into the expansion reservoir, preventing hoses, the radiator, or gasket seals from bursting under high pressure. After the engine is shut off and temperatures drop, a vacuum forms inside the system; the vacuum valve then opens and draws the fluid back from the reservoir into the main circuit, keeping the coolant level stable.
Through this “breathing” cycle the cap keeps outside air out, preventing coolant oxidation/acidification and corrosion of metal components, thereby extending the life of the water pump, radiator, and other parts.
Thus, on both E60 (5 Series) and E90 (3 Series) the cap serves as a safety relief valve, a one-way check valve, and a pressure-tight seal that elevates the boiling point—a triple safeguard. Once the spring weakens or the sealing ring cracks, you’ll see cascading faults such as frequent overflow from the reservoir, erratic coolant temperature, and air entering the cooling system, so timely replacement is essential.
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