About 6000 miles on the 2012 Voyager and I thought I may be running too lean, so I pulled the plugs to check it out. The plugs looked fine but I replaced them anyway since I already had it apart and purchased new ones (OEM spec NGK). I buttoned it all up and went for a ride. Ouch, horrific spark knock from the rear cylinder. I removed and inspected the rear plug and saw nothing wrong with it. So I double checked the gap, installed and torqued to spec. This time I ran it up before putting the tank back together and the knock was still there. So I pulled it out, then cleaned and gapped the old plug and put it back in. No knock, it runs great. I suppose the plug could be bad, I didn't have another one to try. But that seems unlikely with no sign of physical damage. Any ideas??