If the Engine was a New and you drove it a month or so it would have Carbon in it. It is the nature of the Beast.

If every thing is functioning normally the Carbon builds up to a point and stays that way until some abnormal conditions causes more of a build up.
And, if you have an abnormal condition you should fix that.

Other Members who have tried it claim that Water Injection cleans stuff out. I have no personal experience with that so you would have to look up some threads on that.

Freeing up stuck Piston Rings might be different than cleaning out the Carbon from the Combustion Chamber.

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84 300D, 82 Volvo 244Gl Diesel