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NGK does not recommend adjusting their Iridium plugs, I would check the gap before installing but if I found them out of spec I'd return them : This is from their site:
"The re-gapping of fine wire spark plugs that have very small diameter platinum or iridium alloy electrodes is not recommended due to the risk of damaging the electrodes. Virtually all NGK spark plugs are set to the correct gap for the catalogue applications at the point of manufacture."
This video is from NGK training:
 

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2017 Accord LX 6MT Modern Steel Metallic
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My last couple plug changes were all using the NGK Ruthenium plugs. All the cars are running well (Forte, CRV, Renegade). One thing to watch out for with some the Iridium plug versions is to make sure they have some version of better metal on the ground electrode also. Way back I had switched to Iridium in my Corolla as they were the latest/greatest (and expensive) but I wanted better performance/mileage. 30k later car started running a little worse. The ground electrodes wore out I think more than the $2 NGK or Denso it came with BUT the center electrode was great. It wore to the point of creating a much larger gap. After that I switched to Denso Platinum TT and it was good for at least 75k when it got totaled, but ran great.

OK, strange thought but while reading one of the other threads. Normally not until 100k but if you are having a valve issue on that one cylinder it could cause misfires also.
 

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My last couple plug changes were all using the NGK Ruthenium plugs. All the cars are running well (Forte, CRV, Renegade). One thing to watch out for with some the Iridium plug versions is to make sure they have some version of better metal on the ground electrode also. Way back I had switched to Iridium in my Corolla as they were the latest/greatest (and expensive) but I wanted better performance/mileage. 30k later car started running a little worse. The ground electrodes wore out I think more than the $2 NGK or Denso it came with BUT the center electrode was great. It wore to the point of creating a much larger gap. After that I switched to Denso Platinum TT and it was good for at least 75k when it got totaled, but ran great.

OK, strange thought but while reading one of the other threads. Normally not until 100k but if you are having a valve issue on that one cylinder it could cause misfires also.
NGK Iridium IX or Denso Iridium Power are only precious metal electrode. If you want long lasting from NGK, you want Laser Iridium or Denso Iridium Long Life, which have precious metals on both the electrode and the ground. Both of the latter are the plugs to get 100K from.
 

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Discussion Starter #24
My last couple plug changes were all using the NGK Ruthenium plugs. All the cars are running well (Forte, CRV, Renegade). One thing to watch out for with some the Iridium plug versions is to make sure they have some version of better metal on the ground electrode also. Way back I had switched to Iridium in my Corolla as they were the latest/greatest (and expensive) but I wanted better performance/mileage. 30k later car started running a little worse. The ground electrodes wore out I think more than the $2 NGK or Denso it came with BUT the center electrode was great. It wore to the point of creating a much larger gap. After that I switched to Denso Platinum TT and it was good for at least 75k when it got totaled, but ran great.

OK, strange thought but while reading one of the other threads. Normally not until 100k but if you are having a valve issue on that one cylinder it could cause misfires also.
I don't think I have a valve issue because the power is still really strong when running right. Also, the exhaust is clear at idle and doesn't have any significant smell to indicate a problem nor any startup smoke or other exhaust issue I can see. If I had a valve issue, I would expect it to stay and not go away. However, it is possible for some VTEC related issues to be the cause.

I'm still thinking it is a spark plug and/or an Oxygen Sensor problem.

NGK Iridium IX or Denso Iridium Power are only precious metal electrode. If you want long lasting from NGK, you want Laser Iridium or Denso Iridium Long Life, which have precious metals on both the electrode and the ground. Both of the later are the plugs to get 100K from.
At this point, I'm thinking NGK Laser Iridium as they are more widely available though, Denso would be a good option too. Saving a few bucks on the spark plugs is a false sense of 'economy' to me personally.
 

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To answer the question on the O2 sensors. I would replace both. Logic being if one has gone bad the other is or will be bad soon. Why not do them both and move on with peace of mind. They generally don't cost that much. You might also try a code reader/sensor reader like Blue Driver to check the operation of your O2 sensors.

My '15 EX-L is also at 85,000mi, so far all's good. I do plan to do the plugs at 100,000 along with all the other items like coolant, serpentine belt, tensioner, brake fluid, trans fluid (every 20,000 mi for my CVT) and clean the PCV valve and throttle body.
 

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From 'Googling', I see these 3 spark plugs recommended most for my 2015 Accord SE I4-MT. I have an intermittent misfire again on cylinder #4 (P0304) but, I'm questioning blindly replacing the coil again after 20K~30K additional miles which are now at ~85K. The fuel injection system was professionally cleaned at ~65K miles so, I'm thinking a fuel injector issue is unlikely.

I likely have either another bad coil or some other 'wiring' issue or, the spark plug itself is becoming marginal. At ~85K miles it is time for new plugs anyway so, that took me down that rabbit hole!

Is there anything to suggest one of these spark plugs or a different model is better or significantly cheaper with essentially the same performance and longevity? The misfire sometimes is completely gone and sometimes is present. It generally is more prevalent when cold but, also occurs after getting fully warmed up. It will 'skip' occasionally or continuously cut out at ~3200RPM like a LIMP Mode is kicking in. With this intermittent nature of the miss, I am a bit puzzled as to the true cause and source of the issue. This takes me to coil-on-plug #4 again with new spark plugs.

With spark plug installation, do I need to do anything special like anti-seize on the plug threads or something else? It looks like torque is generally recommended at 11-14ft/lbs so, anything else to be aware of or consider?

I'm open to other possible causes and things to look at but, spark plugs seem reasonable at this point so I'm starting there with almost anything else I do. What am I possibly overlooking with this intermittent misfire?

Thanks!
Sid
Do you have a scan tool?

If yes, check Mode 6 and see how many times the cylinder is misfiring. Also check freeze frame data to see if it is misfiring in the morning?

Also what is the MAP sensor reading? And is it stable?

You car is a 4 cylinder, they have timing chain, not belt.
 
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