I had a similar heating problem with my '84 500SEC when I first got it.
Did not boil out any water, but temp. guage ran higher than normal.
Not sure of your location, but here in California, a smog shop indicated I may have an ailing cat. So as a cheaper fix, I first replaced the O2 sensor.
Car heat problem solved. Cat ok. Just FYI

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Selling used Mercedes parts on Ebay...always changing...
cdplayer51 take a look!
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