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Is there any white milky liquid visible under the oil filler cap? That would indicate water in the oil.
You made no mention of a low coolant warning. Is the coolant low? If it's not low, it's not leaking anywhere. It should run just fine with the coolant low enough for it to cause a warning, but not overheat. An N55 does (had a leaky coolant cap) an N52 does, (had leaking o-rings or hoses) and an N62 does (valley pan tube gasket).
If the coolant level is fine and it's overheating, it's almost certainly a waterpump or thermostat issue. Are there any fault codes in the DME? There's at least one for the waterpump, re: communications failure.
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