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 and the ignition coil it was attached to (the ignition coil 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.

 

Didn't you already make a separate thread that was answered by others already? I think we all agreed the plug came loose and was ejected from the cylinder head. Did you torque the spark plugs down as specified using anti-seize? If you didn't use anti-seize then your torque value should have been adjusted accordingly. What that number is, I am not sure.

 

Thank you for the feedback. Yes, I did make a separate thread but was unable to provide photos that were asked for. I finally made it back to the vehicle (in a remote area) and was able to take these photos.

I did not replace the spark plugs myself at 97k... I paid a trusted mechanic to do it.
I'm a little surprised that the consensus here is that the spark plug was ejected- would the spark plug electrode really be pressed in like that if the plug were simply ejected/blown out? It's (obviously) really deformed, as shown in the photo. The plug could have become loose little by little between 97k and 138k... but that doesn't explain the deformity.

 

By looking at the pic of the plug, it looks to me like the plug was loosening/ un-threading from the head. The plug threads that were the last to come out of the head were the ones distorted/ damaged. That would mean the plug was traveling away from the piston as it un-threaded and didn't come into contact with it. Unless a rod broke, it's not probable that the piston came into contact with the plug anyway. As the plug got to the last few threads on it, coupled with the escaping compression and heat around the plug, and the aluminum head, the heat looks like it factored into the distortion of the plug threads as it blew out. Since the coil is attached to the plug, that would result in the coil breaking off with the expulsion of the plug.

You mentioned the mechanic changing your plugs. Did he know the updated 20 lb. torque recommended for the plugs, or did you tell him?

 

@56chevydan That's some fine detective work!

For OP, maybe this will help explain our apprehension about your mechanic's conclusion:


You may well know the above already. I don't mean any offense. It's just.. if that pin that held the piston on came out, or if the rod came off the crankshaft.. stuff would get goofy real quick. like, seized engine. sh*t, you could prolly just rotate the crankshaft by way of the harmonic balancer (biggest pulley) and put this whole matter to rest.

If you were to hear a bunch of clanging and clattering, or it won't rotate, I'd be super interested.

Sorry for what you went through :/ Just looking at how that plug got smooshed I could see how one would think it contacted the piston.

 

It is very obvious the plug was bouncing around after it backed itself/or got ejected out of the cylinder. Everything is mushroomed and deformed in a way that points to it being pushed around the the compression stroke of the piston. I mean I guess you could have dropped a valve and that could have caused some issues as well, but because the only info you provide is pics of the plug, no pics of the cylinder head or top of the piston, the only conclusion we can come to is based on the information given.

If you aren't satisfied with this, then tear the engine apart and post pics of everything damaged.

 

The plug hit something in the engine bay when it came out. It was ejected hard enough to break the plastic on the ignition coil mount. You can bend that plug electrode by hand, it's not that strong.

I can't imagine a mechanic telling me a piston must have hit something in the cylinder, but only that one time. The piston has a fixed travel, from which it won't deviate without grenading the engine. If it was not able to clear the plug there would be major damage. The only way that piston would crush a plug like that is if they put one in that was just slightly too long ie the wrong model plug, in which case, all four plugs would be crushed.

 

Why don't you ask the mechanic if the spark plug was found out of the head or if it was still wedged in there.

Dropping/breaking a valve or a destroyed piston could whack the spark plug as the pieces bounced around the combustion chamber. If the spark plug was found out of the head, then as mentioned it could have been damaged from flopping around the engine/engine compartment.

Best case scenario is the threads let go and the plug popped out. That would bring it down to a somewhat manageable repair...versus a grenaded motor.

 

None of this would have happened if you'd just bought a diesel.