OK, Here is what I have discovered.

With Heater OFF:
Coolant Back-Flows into Washer Fluid Reservoir
If I remove expansion tank cap, back-flow is very rapid.

WIth Heater ON:
Washer Fluid Reservoir Drains into Coolant system.
If I remove expansion tank cap, drainage is very rapid.

After Engine Shut Down:
Some limited coolant back-flow into washer reservoir, and sucking sounds at expansion tank.

The washer fluid heater coil is fine. A cylindrical object -- perhaps DHA's mentioned thermovalve?? (SEE PHOTO) that it connects to is the apparent leaky culprit, as it is where I see the back-flow/drainage occuring. Or perhaps it is where that object connects to the external hose/tubing.

So, by not keeping the heat on, I have unwittingly been draining my coolant.

So, how do I remove this reservoir, coil, and Thermovalve. It seems like an easy fix, even if I need to buy a new part or two.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black