If I start using premium fuel after 80k miles, will there be a release of carbon deposits and would this damage the engine?

My Avensis has done 82,000 trouble-free miles on supermarket petrol. If I now start using Shell high octane fuel, will I get a sudden release of carbon deposits and would this damage the engine? I have noticed a tendency for lumpy running when holding a low gear.

Asked on 3 May 2020 by Bob Browning

Answered by Dan Powell
It's theoretically possible that the cleaning properties of the premium fuel will dislodge some of the deposits within the engine. However, I would recommend getting to the bottom of the lumpy running of the engine in gear one and two. Have the spark plugs ever been changed? If not, they are probably worn out and causing the issue.
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