I have 2004 530i 113k mil. I replaced the spark plugs myself. While doing so I noticed char marks on my ignitors in 2 cylinders. I noticed some marks on the inside of the valve cover too. Looks like part of it is being deteriorated. I was no oil on the ignitor or on the spark plugs. What could be causing this? How do I fix it?

I replaced the plugs with NGK Iridium. I read those are some of the best plugs because they burn hotter.