BMW E39 expansion tank and cap
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Why does the E39 expansion tank leak?
The tank typically cracks at the sensor opening, and the fix is to replace the tank. If you replace only the gasket under the tank, the tank often leaks again soon, so replace them together.
What pressure is the expansion tank cap?
The original cap is 2 bar. For cars over 10 years old a 1.4 bar cap, part number 1 742 232, is often sold. A faulty cap can let coolant out, but the cap alone does not solve a heat problem.
How is the cooling system bled?
Bleeding is done on a cold engine: fill the expansion tank, drive a loop with the cap on, let it cool and check the level. A hot system must not be opened, because the coolant boils when the cap is opened.
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