When a spark plug starts to work its way out, there’s often a very noticeable sound from the engine compartment. Usually it starts as a “pffft” sound, later, as it opens more, becoming a “popping” or “banging” sound. Can be heard at idle rpm.
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The spark plug wells are sealed by the ignition coils.
I would be concerned that the plug was cross threaded in the first place. The original mechanic forced the plug in, damaging the threads in the head and not getting the all the way in. Then he broke the wing tightening the coil down as the coil would not go all the way down because of the plug not being fully seated. The electrode could have been damaged during hte install as well
It took a while but that is why the plug eventually blew out. I would inspect the threads before inserting a new plug. If they are damaged, make sure to chase them with a roll forming die and not a thread cutting die. A roll forming die, also know as a thread restoring die, does not remove any metal. It uses pressure the remold the threads. A roll forming die can actually leave the threads stronger than when new where a thread cutting die would leave them weaker.
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None of my cars have that config. OP’s case, may have to adapt, use mechanic’s stethoscope or length of garden hose to listen to each plug area if wanting to monitor if plug is starting to come loose. Personally I’ve been replacing my own car’s spark plugs for 40+ years, never had a plug come loose.
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Hope you folks are right but I’m just having trouble picturing the plug bouncing around and how that electrode got bent. Just seems like something hit it and pushed the plug up, breaking the threads and coil in the process.
Story. Like I said before I bought my generator from a pawn shop and it looked so new the muffler was not even discolored. Got a deal for $200. Got it home and wouldn’t start but when I pulled the plug it was the same thing with the electrode bent over. New plug and all was fine. Never figured out why. It’s almost like too long a plug was used at the factory but seems to me I replaced it with the same number.
Any chance the plug was mis numbered and too long I wonder?
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The spark plug is unrestrained in the spark plug well, the piston provides rapid pressure and suction. Place your thumb over a spark plug hole while cranking an engine.
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I have to also agree the plug worked itself loose and blew out of the spark plug hole.
If you look at the threads at the end of the spark plug, it looks like the threads in aluminum head gave out and the plug blew out.
In the old days, you usually would find the plug wire dangling from the engine and maybe a spark plug, part of a spark plug, or no spark plug at the end.
But with the coil-on-plug design the plug sits in a well, with coil plugged is onto the plug, and then the coil gets bolted down to the valve cover.
So now if the plug blows out, it’s not going to just go flying with wire attached, but instead pop out of the hole, the bolted down coil is going to stop it from flying, and cause the plug to be forced back down into the well.
And when that happens, there’s no telling where the ground electrode is going to strike the head.
Tester
Maybe while cranking it slowly by hand, but on the starter motor or running, you could be injured by the vacuum.
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It’s compression though not vacume. Haven’t done a car in years but the lawn mower was about 80#. But yeah, it would push your thumb off.
During the power stroke the valves are closed and the piston is pulled down.
Those kind of mysteries bug me. I find myself trying to conjure up some reasonable explanation and revisiting the mental exercise periodically over the years. It’s a disease 
Here’s my thought- I haven’t bought too many spark plugs over my lifetime but I found one new out of the little box with the ground electrode bent tight against the center electrode like it had been dropped. It was a box of 8 and the one with the defect was in the middle of the larger box. I concluded it had been dropped at some point during manufacturing/packaging or some bonehead returned them after dropping one.
I’m thinking whoever built that generator dropped the plug and just installed it anyway. Whoever bought it figured it was junk without investigating- who would sell a new generator to a pawn shop? Someone that knows little and doesn’t invest much time… Those are great stories to hear- winner, winner, chicken dinner!
Because it looks like the last few plug threads contain aluminum from the threads in the head, you need to inspect for the number of remaining good threads in the head. A rule of thumb is the three threads closest to the surface being tightened against (the end opposite the plug tip) will carry the majority of the load, and 5 threads will effectively carry all of it, this is because as the plug is tightened it stretches enough to leave the threads near the end effectively unloaded. If fewer than ~ 5 good threads are engaged (ask an engine rebuilder to be sure) you might look into having a helicoil (thread insert) installed in the head - unless you’re familiar with the process this might be a job for an automotive machine shop…
If a new plug doesn’t screw in easily you’ll need to reform the threads in the head with a thread rolling (reforming) die, as others have mentioned. If lubricant is used with the die you can clean it up with a solvent soaked rag wrapped around a dowel, to avoid potential over torquing problems. If your plugs use crush washers, you can tighten until feeling the crush and not worry about exact torque, but if lubricant remains the next person might not know to do that. The shop that installed the plugs should be told about this, by rights they owe you a repair, but there’s also the question of competency and whether you can trust them to do correct it properly.
I hope the op comes back to update us on the results.
My theory on the generator is that there was a weather issue in that town if I remember back. So a guy bought it and didn’t need it. Jimmied the plug and returned it to Home Depot or whoever saying it didn’t work. Hd doesn’t fix anything and just dumped it on the pawn shop. Or else the original owner pawned it and expected to go get it again, making sure it wouldn’t get sold. But the plug had enough carbon on it that it had run for a little while, thus I’m thinking a mislabeled plug that got whacked by the piston maybe. But in the end who cares?
Thank you all for the feedback. This incident occurred in a remote, rural area at midnight on a Saturday night. I slept in the car for two nights before being able to get it towed on Monday morning. It was towed to the nearest town and a mechanic who looked at it said the piston was hitting the spark plug and the cylinder is “dead”. He said the cost to repair would almost certainly be higher than changing the engine out with another used engine from a junk yard. Knowing that it would have cost an additional $650 to have my car towed to my city (and my own trusted mechanic for a second opinion), I decided to follow this mechanic’s advice and had the engine traded out with one with 15k fewer miles. Total cost $1,600.
In hindsight, according to the consensus here, the spark plug simply became loose over time and was ejected when on its last threads. So the root cause of all of this is the mechanic who installed the spark plugs at 97k, (four years ago). Strange because the mechanic who replaced the spark plugs at 97k was a trusted mechanic who I’ve used for 15 years without incident, but people can make mistakes/be careless. I do wonder if the ejected spark plug caused damage in the cylinder (given that I was going 75 mph when it happened). Perhaps something was damaged in the cylinder.
Lessons learned:
- Be extremely picky about who you pay to replace spark plugs. I normally do something like this myself, but on this car it takes a lot of effort (part removal) just to gain access to the spark plugs.
- Be skeptical of what an unknown mechanic tells you (even if exhausted after two nights of little sleep) and be willing to shell out many hundreds of dollars to have your car towed to a mechanic you know and trust.
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Lessons learned:
- People are human.
- Humans are not perfect
- Don’t believe everything you’re told.
If the plug was hit by the piston the electrode would have been flattened into the insulator or at least show a considerable flat spot on it. Neither of which happened.
PSI??? The torque recommendation for my Subarus and Toyotas is around 15 ft. lbs.
Just because you install a spark plug perfectly and literally by-the-book, does NOT guarantee it won’t come out with pieces of aluminum from the cylinder head attached, when it comes time to remove it a few years later
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It doesn’t seem everyone here concurs with that diagnosis. There’s a measuring gadget your shop could use to confirm the piston is coming up higher than it should be in its travel. Dial-indicator. Might not be 100% conclusive, b/c this measurement is made with engine not rotating. Play in connecting rod bearings combined with centrifical force might allow piston to move higher when engine is running. But usually there’d be unusual engine noises with that much bearing play.
OP replaced the motor for $1600 in a town, far from home? Hard to believe. Honda fit motors are hard to find.
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