Coolant expansion tank
Provides space for coolant expansion and a filling point.
In the engine bay or near the coolant circuit.
The tank accommodates volume change and may carry a level sensor; the cap manages specified pressure.
A buffer reservoir in a pressurized water loop.
Visible seepage, low-level warnings or cracked fittings need system inspection.
Fluids use vehicle-specific schedules. Seals, pumps and heat exchangers depend on age, condition and operating history.
Simple seals and filters can be accessible; an integrated pump, buried housing or overheating damage can make repairs more involved.
BMW coolant reservoirs and caps must match the specific circuit; pressure specifications vary.
BMW combustion models; additional coolant circuits also appear in hybrids and electric BMWs.