What happened to this spark plug/ignition coil?

I was driving 75mph on the interstate and my check engine light suddenly came on, and the engine started misfiring really, really badly. I immediately pulled over & got my Fit towed to a mechanic. The car had been running perfectly, I bought the car new & have always changed the oil when the maintenance minder says 20% oil life remaining, & I changed the spark plugs at 97k (this incident happened at 138k).

The mechanic’s computer narrowed the issue down to one particular cylinder. I’ve attached a photo of the spark plug in that cylinder (really deformed). The ignition coil it was attached to had a major crack where it bolts into the engine. What would cause this to happen? The mechanic says a piston must have hit the spark plug for this to happen. Any insight would be much appreciated. How much might this cost to repair?

So, mechanic sees bent plug and goes huh? What does he suggest doing next?

Looks like the spark plug came loose and was bouncing around in the spark plug well.


I suggest something came loose in that cylinder and bashed the plug. More investigation is required. A compression test comes to mind. A boroscope look inside, too.

Is that spark plug the exact model and gap recommended by the carmaker? See your owners manual or underhood sticker to ascertain this.

Anything else is a mistake or an experiment, with consequences that may not be desirable.

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I agree. Hopefully when the spark plugs were replaced there wasn’t any anti-seize placed on the spark plug threads.

Why not just install a new spark plug and coil and see what happens? A lot quicker and gives me just as much information.

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Not to disagree but sure looks like something slammed against the electrode and dislodged the plug. Timing belt would have bent the valve not the plug? Some ingested debris?

Those threads are rather munged up. How do the threads in the head look?

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If the plug threads in the head are shredded, the compression tester won’t thread in… or mine won’t… and question will be answered.

Since the plug looks like it was hit and the coil was broken, too, I gotta think there is either stuff bouncing around in the cylinder or the threads failed.

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Spun rod bearing perhaps.
Malice in the combustion palace.

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It’s just the last few threads that are damaged. Looks to me like the plug was loose, and when the last 2 threads gave way the plug tried to exit the engine and broke the mounting ear of the coil. Pretty common to see on older Ford engines. The ground electrode was bent banging around the spark plug well.

Chase the threads and throw a new plug and coil in.


Debris in the cylinder can easily damage the electrode, but to eject the spark plug from the cylinder head is nearly impossible. The tread damage to that spark plug cannot occur while properly installed into the cylinder head.

Friction on a connecting rod bearing sufficient enough to cause the locking tabs to fail will not damage a spark plug.

That spark plug unscrewed from the head and was bouncing up and down in the spark plug well. Don’t expect the person who installed those spark plugs to admit that they were not tight enough.


I’m guessing Nevada’s diagnosis is correct. The way to prove it, install new plugs and coil for that particular plug as required, then turn engine by hand to make sure it turns freely within any snags at least two times around. If so, I expect it will crank, start and run as before. If problem recurs, ask shop to double-check valve timing. If your Fit uses a timing belt, might be a good idea to ask shop to visually check its condition too.

Given that the thread damage is steadily worse as you go up, I agree with this.

Thank you everyone for the feedback, it is much appreciated.
It’s pretty obvious what the consensus is.
I generally do as much maintenance as possible, but unfortunately it’s a real pain to get to the spark plugs in a Honda Fit… that’s the reason I paid someone to replace them at 97k. And because the plugs are not easily accessible, going forward it will be a challenge to make sure they’re not getting loose.

What happens if anti-seize is on the threads?


It says PSI!!!
I think that may be one of those compiled articles done by people who often off shore. They often know nothing about the topic they are writing about. They copy and paste content to create articles for ad revenue or some marketing program.

It reduces the torque required to seat the plug properly, which can result in overtightening and deforming the threads or gasket. It can interfere with heat transfer from the spark plug to the head, impairing spark plug performance. It can allow a partially loose plug to vibrate its way loose in the head and come out (which is what happened in the picture posted above). It’s messy.

In short, there are several reasons not to use it and no reason to use it.


I believe you don’t use it anymore because they all have it from the factory. Adding more is unnecessary and changes the torque.