Welcome to the Nissan Patrol forum. To post a question and to see less adds on the forum then you will have to register
first. We are an easy going friendly forum so join in the conversations and feel free to ask any questions.
Become a forum sponsor for only $20 and see no adds with faster page loading times and many extras benefits.
I would pull the plug and simply replace it and monitor. I seem to remember build up can occur from the 'corona' of the spark itself if the plug gap or plug itself are faulty, gap usually being too wide. Maybe jump onto the web and do a search on plug defects and condition analysis - might be a lead, and save hours of heartache also.