Best way to find a coolant leak that isn't glaringly obvious is to go to one of the mass-marketers like Auto Zone and rent a cooling system pressure tester.

The test is performed on a cool motor; pressure is applied and the coolant will begin to flow from wherever it's leaking. Lots of yack about this using this sites SEARCH facility or googling it on the WEB.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"