Our 2004 285,000 mile 3.9 Windstar came in running rough. Code for #4 misfire was stored so I pulled the plug. Ugh: broken insulator. Pulled #5 and it was actually worse. Pulled #6 and it looked good. Picture follows.
So can you spark experts help out a Diesel head? What is going on? I have looked at charts on reading spark plugs and it points to - most likely - running lean. I think I replaced the intake manifold seals on this one a few years ago, but if I had a leak on the runners to just these two cylinders, would that cause it to run lean on just those two?