If you're running only water at the moment, this is my suggestion:

1. First get the coolant capacity (qts) of your system.
2. Drain all the water you can using the radiator drain and engine block drain.
3. Close drains.
4. Add 50% of the capacity with antifreeze, the remaining charge with water.

This is just the bare minimum that should be done. There are many other steps that can be added to this procedure - some of them can get anal - but this should get you started.

Running only water is never appropriate for a cooling system for many reasons.

If you want to read more archived information on cooling system maintenence, search using keywords "demineralized", "distilled water", "coolant flush".

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95 E320 Cabriolet, 169K