Replaced a full set of spark plugs today on a 15 year vehicle with 100k miles. I looked at each plug for abnormality and did not see anything, so I did not mark them. Only after the entire job was done did I carefully examine the plugs, which I foolishly didn't mark with cylinder numbers (I had considered it but was in a hurry and the job was already taking too long due to the brilliant engineers designs). Drove 5 miles, and it purrs, and the misfire is totally gone. I sat at idle for several minutes, and the engine sounds and feels perfect.
Tonight I examined the plugs more carefully. I suspect these are original factory plugs, with 15 years and 100,000 miles. (Manual states to replace these at 90k, but they are platinums rated for 60k-100k. So given the miles and age, the absolute end of their designed lifespan.) They are all uniformly worn, gray/brown, some fouling, nothing alarming except 1 plug. One plug, and I'm not sure which cylinder now, has a broken missing section of the insulator, which is the ceramic section that insulates the electrode. Probably 2-3 millimeters length appears to have broken off and presumably fallen into the cylinder. I've read various opinions around the internet ranging from it'll get ground up and passed thru the system harmlessly, to it will bend valves, score pistons, and blow a head gasket and destroy the engine.
I don't hear any rattling or other noises and the engine runs fine. My concern is that I'm being naïve and overly optimistic. My theory, at some point maybe recently but maybe long ago, the insulator broke off and was consumed and passed thru the engine and expelled out thru oil or exhaust. It might have come off in one piece or slowly in small pieces. This also caused a slight misfire on the plug and cylinder. The engine runs better with new plugs now. Hopefully it was ground into dust and the oil or exhaust carried it away? I'm hoping it's not suspended somewhere, a ticking time bomb waiting to jam a valve and score a piston...
I could get a borescope, and that would require pulling every plug again. Or remove the valve covers and inspect. Or just move on with life and accept whatever happens will happen. I'm inclined to do the latter and think if any damage was going to be done, it would have already occurred.
Tonight I examined the plugs more carefully. I suspect these are original factory plugs, with 15 years and 100,000 miles. (Manual states to replace these at 90k, but they are platinums rated for 60k-100k. So given the miles and age, the absolute end of their designed lifespan.) They are all uniformly worn, gray/brown, some fouling, nothing alarming except 1 plug. One plug, and I'm not sure which cylinder now, has a broken missing section of the insulator, which is the ceramic section that insulates the electrode. Probably 2-3 millimeters length appears to have broken off and presumably fallen into the cylinder. I've read various opinions around the internet ranging from it'll get ground up and passed thru the system harmlessly, to it will bend valves, score pistons, and blow a head gasket and destroy the engine.
I don't hear any rattling or other noises and the engine runs fine. My concern is that I'm being naïve and overly optimistic. My theory, at some point maybe recently but maybe long ago, the insulator broke off and was consumed and passed thru the engine and expelled out thru oil or exhaust. It might have come off in one piece or slowly in small pieces. This also caused a slight misfire on the plug and cylinder. The engine runs better with new plugs now. Hopefully it was ground into dust and the oil or exhaust carried it away? I'm hoping it's not suspended somewhere, a ticking time bomb waiting to jam a valve and score a piston...
I could get a borescope, and that would require pulling every plug again. Or remove the valve covers and inspect. Or just move on with life and accept whatever happens will happen. I'm inclined to do the latter and think if any damage was going to be done, it would have already occurred.