Open the hood, remove the cap on the water reservoir, fill the reservoir till the top while the (recently used) engine is running and REV it a couple of times. If the water comes blowing/boiling/sputtering out of the water reservoir, you have a leaking head gasket (causes the engine temp to float around).
Watch your head incase the (hot) water does come blowing out of the reservoir!

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)