Surely when you replaced the water pump all of the old coolant will have drained out of the block and the disconnected radiator hoses. Happened to me on my recent water pump replacement on my 2.8 litre M104.

All you'd have to do now that it's reassembled is use some citric acid flush and refill with the correct coolant again.

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Steven Harrison

1993 300E2.8 - 196K
2001 E320 4MATIC - 86K