Here is a spark plug with clearly visible whitish tracks showing where high voltage was draining to the ground instead of jumping across air gap producing a spark. Cylinder where this spark plug was installed had misfire under load (on acceleration) at the mileage of just a bit over 60 000 km on Ford F-150 3.5 EcoBoost. It is rather unusual to see spark plugs condition like this at that mileage, probably poor fuel quality was the case of it.

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Anyway I would recommend replacing spark plugs at around 60 000 km even if this mileage is way below manufacturer recommended interval for spark plugs change.

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