I have a 2014 Explorer Sport with a little over 29000 miles on it. I'm preparing to put a tune on it, so decided to swap the factory installed plugs with new ones that I carefully gapped. In removing the plugs, the middle rear one came out looking quite corroded, while all the rest looked to be in great shape. Here's a picture: What would cause this? Bad grommet on the top of the coil pack? I inspected and cleaned them all and didn't notice any tearing or such. Also didn't notice any water coming out when I shot some compressed air into that well prior to removing the plug. Thoughts?