Coolant temperature display & intermittent engine chugging

Hello. I have a Mercedes E350. A week or so ago I started to get coolant temperature warning lights (frequently but not continuously). I topped up
the radiator with a glass of water but the warning light is still lit up, and today the engine started "chugging" intermittently. Are the two symptoms connected?
Thanks for any input.
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Sebastian Drayton wrote:

Coolant isn't just water. You might have bigger problems. Take it in - soon!
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