So, I tried posting this to the r/AskMechanics the other day, and got zero response (maybe posted at the wrong time, too wordy, who knows) so, I figure I will check here for any knowledgeable folks that might have some wisdom to impart.
Car: 2010 Mazda 3, 2.5L I4 Duratec, 162K Miles
Alerting issue: check Engine Light Flashing, P0304 (Misfire Detected, Cylinder 4)
To check for a failing coil, I swapped 3 and 4. No new codes.
I then checked the plug and immediately noticed it was loose (as in next to zero torque required to pull it). When I got the plug out, I found lots of carbon all the way up the threads and, both the electrode and anode tips were gone.
This is the second time I've found the cylinder 4 plug loose (the first was in my first routine plug change cycle, back in Aug 2024). The threads on the cylinder don't look perfect but, they don't look cross threaded. A new plug definitely starts loose and doesn't present much torque until it shoulders, and I've been using a Torque wrench to the center of the specific range (according to the factory service manual) for any and all fasteners, ever since I bought the car. I was able to get the new plug torqued in to the upper range of the torque spec.
Given that I've only ever changed the plugs once before, and in that time, I found the same plug loose, I suspect that the manufacturer over torqued plug four and, stretched the threads a bit, allowing it to loosen over time, which, I suspect allows for even more wear due to a less than ideal heat transfer from the plug and, a less than ideal electrical connection.
Here's something I realized- For many years, (even prior to my plug change) I've had intermittent startup issues. A 'normal' startup for this car is what I experienced immediately after changing the plug- Key on, and within two seconds the engine is running normally, no need for throttle. The intermittent issue was where sometimes I would have to key on for twice as long or longer, often using gas to start the car. Not reproducible on demand. I asked a shop about it once, and they found nothing. Is it possible this is related to plug 4? Where it gets a loose and doesn't create a good spark and, it happens to be the first cylinder to spark on a given startup.
I'm currently driving the car with a regular and a torque wrench and plan to periodically check the plug. Should I just re-thread the cylinder with a spark plug thread repair (heli coil or similar) kit and be done with it?
Car: 2010 Mazda 3, 2.5L I4 Duratec, 162K Miles
Alerting issue: check Engine Light Flashing, P0304 (Misfire Detected, Cylinder 4)
To check for a failing coil, I swapped 3 and 4. No new codes.
I then checked the plug and immediately noticed it was loose (as in next to zero torque required to pull it). When I got the plug out, I found lots of carbon all the way up the threads and, both the electrode and anode tips were gone.
This is the second time I've found the cylinder 4 plug loose (the first was in my first routine plug change cycle, back in Aug 2024). The threads on the cylinder don't look perfect but, they don't look cross threaded. A new plug definitely starts loose and doesn't present much torque until it shoulders, and I've been using a Torque wrench to the center of the specific range (according to the factory service manual) for any and all fasteners, ever since I bought the car. I was able to get the new plug torqued in to the upper range of the torque spec.
Given that I've only ever changed the plugs once before, and in that time, I found the same plug loose, I suspect that the manufacturer over torqued plug four and, stretched the threads a bit, allowing it to loosen over time, which, I suspect allows for even more wear due to a less than ideal heat transfer from the plug and, a less than ideal electrical connection.
Here's something I realized- For many years, (even prior to my plug change) I've had intermittent startup issues. A 'normal' startup for this car is what I experienced immediately after changing the plug- Key on, and within two seconds the engine is running normally, no need for throttle. The intermittent issue was where sometimes I would have to key on for twice as long or longer, often using gas to start the car. Not reproducible on demand. I asked a shop about it once, and they found nothing. Is it possible this is related to plug 4? Where it gets a loose and doesn't create a good spark and, it happens to be the first cylinder to spark on a given startup.
I'm currently driving the car with a regular and a torque wrench and plan to periodically check the plug. Should I just re-thread the cylinder with a spark plug thread repair (heli coil or similar) kit and be done with it?
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