Hi Waldi....I've attached a link to a Tech article on the SLK's Cooling system. I think this will help you with pinpointing the issue and addressing it. If you have any further questions, feel free to Comment just below the article and someone will get back to you shortly. Good luck!

Mercedes SLK Cooling Tech Article - Mercedes-Benz SLK-Class R170 (1998-2004) Technical Articles ? Pelican Parts


-Dmitry