My first thought was a chunk of carbon.
Carbon build up from low quality fuel can do that. It can also cause the difference in plug gap, as in smaller pieces hitting and breaking apart inside the combustion chamber.
Do you run a decent grade of fuel? Or run fuel system cleaner?
How did the compression test come out? Did you compare the reading to another cylinder?
Also I believe those Champion plugs are copper core, and should be replaced at 30k miles.
edit: Sorry I just caught the difference in color between the plugs as well. You may want to check on your o2 sensor or for a vacuum leak if the three lighter colored plugs are all from the passenger side.
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Last edited by bfj aka tambil; 04-21-2013 at 04:37 PM.