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The Reason Why Used Spark Plugs are Blackened

Model: (Various brands) Various
Part: Spark plugs

Video tutorial

This video explains why spark plugs become blackened during use. It compares a new spark plug with a used one that is heavily blackened, which leaves carbon deposits on your fingers when touched. This happens because the spark plug has operated at too low temperatures.

Spark plugs have a thermal grade (i.e., a capacity to dissipate heat), and if they dissipate too much heat for our engine and never reach the 450°C necessary for self-cleaning - during which deposits are burned off - carbon and residues accumulate. Another reason could be the incorrect air/fuel mixture: if it is too rich, there will be an excess of fuel, leading to these black and oily deposits; if it is too lean (with an excess of air), then the spark plug will be whitish and worn.

Other reasons may include engine oil entering the combustion chamber due to worn gaskets, or too short journeys that do not allow the engine to reach temperature. When buying spark plugs, choose the correct thermal grade: always consult the vehicle manual for this.


Posted on 10 March 2026 by ScegliAuto


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