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Discussion Starter · #1 ·
This is an open discussion to see if anyone has experience with what is going on here. Here is my experience with cyl5 failing catastrophically (destroying plug, plug threads, and coil):

  • 2 months ago: my brother's 2005 Pilot with 230k miles starts misfiring and pretty soon (couple miles) is undriveable. Cyl5 (front center) is the culprit. After removing the charred and shatter bits of coil and plug, and re-threading the hole, cyl5 has NO compression. I'm currently parting this Pilot out and then will scrap it.
  • 1 week ago: stranger's 2005 Pilot with 150k miles is stuck on the side of the road and I pull over to help out. Cyl5 (front center) is the culprit. The coil is exploded into bits, the radiator fan is cracked, and there is coolant dripping down the engine and radiator. My guess is the coil exploded into the fan and cracked the radiator, causing the coolant to leak.
  • Yesterday: my 2005 Pilot with 200k miles on it. I change the plugs and inspect the coils. All plugs looked the same, had the same gap, didn't look overly-fouled, etc. While I had the plugs out I ran a (dry) compression test. All cylinders around 160-170psi (after about four cranks). EXCEPT cyl5. It was around 140-150psi. I re-ran the test wet in cyl5, and it improved compression to about 160-170psi. However, I ran wet in another (good) cylinder, and that reading was 170psi (but with less cranks, about 2-3). (Note: compression test performed at 9k feet elevation, if that matters). (Note: no codes were appearing at this same time).
  • This old thread: Catastrophic Front Center Spark Plug Fail .

Looks like this is a common problem, and I'd like to avoid a catastrophic failure to my Pilot. So where could the issue be? My theories:
  • Sticky or out-of-spec valves.
  • Loose spark plug somehow allows combustion to leak through plug threads.
  • Busted coil melts itself, eventually leading it to arc at the wrong time at the place (e.g. not at the plug tip, but somewhere in the coil).
  • Damaged piston or cylinder (although I don't know how that would lead to an overly explosive combustion cycle in that cyl).

Thoughts?

--Chris N.
 

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OEM Plugs? Are you torquing the spark plugs to specs or just tightening them? My money is on a loose plug.
 

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Loose plug was my first thought too. Loose coil pack is my second guess. But loose plug probably makes more sense since some of the hot gases from the combustion chamber will seep past the loose spark plug and onto the coil pack. Bad things happen when electrical components get too hot.

On the engine with no compression in the cylinder I’m guessing parts of the spark plug or coil pack, after catastrophic failure, damaged the valves.

There have been quite a few posts over the years for checking that spark plugs are not backing out. I suspect that may be due to improper torque applied to the replacement plugs, since when I changed the original plugs on my wife’s 2005 Pilot last year all six felt much tighter compared to the torque applied (I think it was ~13 ft-lb) when installing the new ones.
 
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I'll make it a triple play and suggest to check and retorque all your plugs to spec as a maintenance item on these vehicles.

(I think it was ~13 ft-lb)
Ding, ding, ding. That is correct. Great memory.
 

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Discussion Starter · #5 ·
Great information guys.

I'm still wary because these were all the same cylinder (including my lower compression one). What are thoughts on the causes of that? Is it a good idea to start with a valve re-adjustment?

--Chris N.
 

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I understand your concern since it’s the same model year and same cylinder. That’s a trend.

If you haven’t adjusted the valve clearances ever it’s well past overdue. If they were adjusted at 105k miles then it’s just about time for the second adjustment. I’m curious if the compression improves on that cylinder after the adjustment. Keep us posted.

I’m also thinking if there is a good deal of carbon buildup on the valves that running some fuel system cleaner before the valve adjustment might be helpful. Either that or take the Pilot out for a spirited drive to get the engine nice and hot (which hopefully helps clean the deposits the valves).
 
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