There may be a lot of reasons.

Have a look to the moulded connector on the coolant temperature sensor ( double sensor on the clutch side, on the head) the wire is red green. It may be damaged (can be obtained as a spare ref 013 545 87 28).


have a look also to the thermovalve on the head

Do not change part without having found the real cause it will lead to waste of money.

good luck

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Sold: E-class, 320CDI, W210, 2000, 225 000km