Originally Posted by wander53
Thank you all for your help with this back in 2019.
It turns out the issue was actually either the PLUG WIRES (most likely) or the SPARK PLUGS themselves. I changed both, and the issue was then completely resolved. I assume the liquid coolant affected the old plug wires and led to misfiring.
Might have actually been heat-related. My '04 W22 8.1 lost a plug on I-15 between St. George and Las Vegas this past summer. The motor was running fine but the ambient daytime temps were well north of 100įF and I was pulling a 20' enclosed trailer. Limped it home to LA overnight on 7 cylinders.
When I pulled the plugs at home I found #6 with clear indications of tracking across the insulator cone. No visible crack but electricity doesn't need a lot of room to flow.
Edit to add: The coach threw a code but not a misfire code, it was actually an O2 sensor code (too rich on right bank). Technically the hole wasn't misfiring, it just wasn't firing across the gap but into the cylinder head. It would backfire something fierce at WOT with all that raw fuel dumping into the exhaust.