So I got an intermittent misfire in cylinder 5 while idling, and I suspected the coil pack, but after switching the coil-pack with another one without the misfire moving, I suspected a plug, so I changed it out, and presto, misfire gone. I'm wondering why this plug failed at 20k miles, I used the OE denso iridium plugs. I should mention that I did test pipes and and exhaust and ran like that for about 6 months (I think It ran with the AFR slightly off) without a tune, then got it tuned all on these plugs, maybe that stressed the plug to premature failure, although none of the other plugs have shown signs of failure yet, and the plug showed normal wear, looked healthy. Did I just get a bum plug? Or should I suspect some other problem condition that caused this.

 

Densos and NGKs generally have very good QC. If the plug looks good, you might try swapping it with another one and seeing what happens.

 

I did replace the plug with the one I bought to test out the coils and now I don't get any misses, so I do believe that was the problem (unless there was some other issue that happened to rectify it's self). Just left trying to figure out why the plug failed so prematurely and wondering if there is another issue at hand I should investigate.

 

When you say "failed" you are referring to misfire only, or fouled and worn electrode on visual inspection?

 

It only had an intermittent misfire, the plug looked perfectly healthy.

 

Call Denso and tell them you have a crap plug and want it replaced under warranty.