very unusual , the only thing i can think of is You are using the wrong coolent. these cars like all Mercedes must use
mercedes coolent.,,, don't beleave me read below or google ,type coolent to use in older mercedes
or just replace the rad every 4 to 6 years

https://www.mbca.org/sites/default/f...reedec04_0.pdf


https://www.googleadservices.com/pag...9aACCFI&adurl=