Advice: Found spark plug with broken insulator.

Joined
Jul 23, 2021
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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.
 
Joined
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Athens, GA
It's long gone, don't worry about it. If it were going to have damaged something it would have within seconds before it got shot out the exhaust valve. If it left whole, it will be sitting on the cat somewhere, if it got pulverized, its gone.
 
Joined
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That ceramic is very hard to grind up, hopefully it flew out long ago. Maybe if you could put a shop vac sealed over the hole, it would pull something like that out
if it is there. Or maybe the insulator got broken before installation and the plug was installed anyway. . That sounds more likely.
 
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Displaced Texan in Mexico City
I seriously doubt that any foreign object will stay in a cylinder for more than a few seconds. If it was going to damage the piston or valves it would have already done its "job". It wouldn't burn off either since ceramic has a very high heat resistance. It wouldn't shoot out the tailpipe either, because the catalytic converter has a ceramic honeycomb. Either way unless you want to disassemble the exhaust from the manifold to the cat, I would leave it alone and go buy a lottery ticket because you got extremely lucky, usually a foreign object in a cylinder=catastrophic failure.
 

CharlesInCharge

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I considered these options. I don't see the value in the time/money/effort in a compression check, and I'd have to do it with all cylinders since I don't know which one had this bad plug. I suppose I could try the shop vac on all cylinders, but that also involves basically re-doing the entire plug job. Both operations almost certainly will result in nothing gained and at the added risk of breaking something.

My big concerns are pretty much confirmed - any damage would have been immediate and otherwise noteworthy, or no damage was done and it's been expelled already. The car is running fine, so compression loss, noises, etc. does not appear to be a problem.

How long would anything, ceramic or otherwise, remain in that firing chamber? Seconds? Minutes? My suspicions are largely confirmed in that it would either have caused damage immediately, or been pulverized and flushed out. If I hit a piece of ceramic with a hammer a hundred times it will be dust. This ceramic might have eroded slowly over 10k miles, or busted off and been pulverized 20k miles ago. I have no inexpensive way of knowing. My only data is the old plug, and the currently well running car.
 
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