Like Carbon in other cars on 1992 Mercedes-Benz 300E

I had a mechanic recently told me he cleaned something on my car that is "like carbon" on American cars. I left a message and he said to call him Monday, but he has not gotten back in touch with me. He said it is round. He did not know what it is called on a Mercdes. Can you help me out?

I have no idea what he's talking about, maybe there is some misunderstanding. Call him back and ask to explain.