Hi, I have a 2013 Honda Fit, just 60K miles on it.

~6 months ago it suddenly "squeezed out" one of the spark plugs, without anything being wrong before. Naturally spark plug and coil were burned to crisp while I got it into service.

It was repaired with a thread insert, and worked well for a long time (several months), but always smelled with gasoline inside the car when stationary. But it worked and drove perfectly well.

1 month ago I brought it back to service to figure out the smell, and they found that the seal on the thread insert was not perfect, and it was leaking fuel/exhaust a bit when running. Very little - not enough to damage the spark coil - but enough to smell.

They re-installed the thread insert, and it stopped smelling, but ~200 miles later it burned that spark plug completely. The thread insert is still tight, no leaking, the ignition coil on the cylinder is fine.

We checked the seals and the cylinder for debris (incl. inside with a camera), did not see anything suspicious, and replaced the spark plug - but it burned it again.

The plug looks correctly inserted from what we can tell here, and the spacing in that engine is very tight - I suspect if we had it too deep it would hit the piston.
Anyway - for some reason that cylinder runs hot and burns the spark plug.

The car shows no error codes until the spark plug is dead.


So, trying to figure out root causes and best next steps. My thinking so far:

1) It runs hot BECAUSE of the threaded insert repair - like spark depth is wrong, or some roughness on the inside of the cylinder. Then this is a cylinder head replacement job, which is not worth it for me on this car.
2) It ORIGINALLY squeezed out the spark plug because it started to run hot. Then after first repair it worked OK for a while because thread was not perfectly sealed, so it never overpressured. In that case what would be the next thing to logically check?
- Replace fuel injector on that cylinder?
- Check valves?
- Exhaust manifold leak?
Any way to diagnose which of these is more likely?

3) Funny third option?

Thanks!