A black line on the ceramic end of a spark plug, according to one mechanic, is normal. Another mechanic suggests the black line might suggest carbon tracking. But who's right?
Black markings and lines on spark plugs may be carbon tracking or "flash over", it is the most prevalent failure with coil-on-plug engines.
Although carbon tracking is usually caused by a path to ground being created over the spark plug insulator by multiple causes, including the following.
A severely eroded spark plug electrode will increase firing voltages to the point where the spark will seek the path of least resistance, which is either through the spark plug boot or down the insulator.
Diagnosing & Fixing
Take out the spark plug and inspect the ceramic insulator for black markings and lines.
If a carbon track is discovered on a spark plug, a carbon track can also be discovered within the spark plug boot. The boots should be changed at the same intervals as the spark plugs if the boot is serviced separately from the coil.
If the boot is only accessible with the coil assembly, the spark plugs should be changed at the prescribed intervals, and the boot and insulator should not be contaminated with dirt or oil. Always check the old spark plugs and boots before replacing them with new ones.