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I had been having some hesitation problems on my 98 Mazda 626 which after a great deal of research seemed to be solved by replacing the plugs:

enter image description here

As you can see, there seems to be signs of flash over / arcing on the number one cylinder spark plug.

After a while the problem seemed to come back, and I couldn't understand why. Then while I was doing my front oil seal, timing belt and water pump I had to pull the replacement plugs:

enter image description here

As can be seen, there seems to be arcing on the number one cylinder spark plug again! ( in addition to what looks like carbon fouling on the number 1 & 4 plugs, but that's a separate issue )

Any ideas what the heck might be going on?

EDIT Nov. 13th 2016

So @FredWilson was right about the boot having a carbon trail in it:

enter image description here

However I couldn't see anything like that in the #4 plug boot. I think I'll try the dielectric grease by itself first just for fun to see what happens.

POSTSCRIPT Nov. 28th 2016

So I pulled out all the plugs, and cleaned the porcelain sections and the plug boots with IPA and 320 grit sand paper to remove the carbon trails, then I slathered some 3M Silicone Paste ( dielectric grease ) on the porcelain part of the plugs and dabbed a little into the boots.

So far so good, the hesitation is gone and the increase in power is noticeable. It seems to have helped somewhat with the idle as well, although that's really a separate and intermittent problem, which I suspect is due to either clogged / leaky injectors or a wonky fuel pressure regulator.

  • What's the mileage on these plugs? – tlhIngan Nov 10 '16 at 19:54
  • @tlhIngan Just a few thousand kilometers at most. They're practically new. – Robert S. Barnes Nov 10 '16 at 19:56
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    Are you using dielectric grease when you installed them? And did you utilize new wires? (Or are these a coil on setup?) – Pᴀᴜʟsᴛᴇʀ2 Nov 10 '16 at 20:11
  • @Pᴀᴜʟsᴛᴇʀ2 The one thing I haven't replaced are the wires. Rebuilt alternator and got a new coil. I did not put any anti-sieze or anything on the plugs. You think the wires might be causing this? – Robert S. Barnes Nov 10 '16 at 20:21
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I think your problem is your plug wires. I think they are leaking along the boot and down to ground (earth). With this loss of energy, you'll also see the large carbon buildup on the business end, due to unburnt fuel deposits. Since you are saying you are seeing the carbon tracking on the number one plug again, this is my reasoning for telling you the plug wires. Since they haven't been changed, this is the most likely culprit.

Also, whenever you put new plugs or wires on, use dielectric grease. Squirt a little into the boot just before you put it onto the spark plug. This will do two things for you. First, it will keep electrical leaks like this to a minimum. Dielectric grease doesn't transmit electricity, so it becomes an insulator. Secondly, it will also keep the boot from adhering to the porcelain of the spark plug. If you ever need to take the boot off the plug for any reason, it will come off a lot easier.

  • I was just reading the NGK page I linked in the OP, and kind of get the impression there is a bit of a chicken or the egg thing here: Is a rich A/F causing carbon fouling which is causing misfiring which is causing the arcing, or is a deteriorated plug boot causing arcing ( and mis-firing ) which is causing rich A/F which is causing carbon fouling? Maybe if this was being causing by rich A/F due to dirty injectors then I would expect to see arcing on the #4 plug which also is fouled? Or maybe it's some combination of the two problems? – Robert S. Barnes Nov 11 '16 at 6:49
  • It says on the NGK page: The plug cable material hardens as time elapses, which in turn reduces the tightness of the cover and insulator, lowering the preventive power for flash-over. I was just thinking that if this is being caused by loosening of the boot lowering resistance, then maybe the problem could be at least temporarily solved by just applying said dielectric grease to the current boots without replacing them? – Robert S. Barnes Nov 11 '16 at 7:23
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    @RobertS.Barnes - It's definitely worth a try. Part of my reasoning is, you are getting the same carbon tracking down the porcelain as you saw on the first go round. The boot is just going to allow electrical leakage no matter what you do, so I'd suspect the dielectric grease is only going to work for so long. There is most likely a carbon trail inside burnt into the boot which is allowing this. I'd suspect the #4 is the same way, just not as bad (based on how it looks). – Pᴀᴜʟsᴛᴇʀ2 Nov 11 '16 at 11:07
  • When the arcing happens on the plug insulator there is also a corresponding carbon track on the inside of the spark plug rubber boot. Look closely with a bright light and it can usually be seen. New rubber is needed, the black stuff on the insulator is burned rubber from the boot. – Fred Wilson Nov 12 '16 at 8:07
  • @FredWilson I wish you'd told me that the last time I had a plug with arcing on it, this has been driving me crazy :-) – Robert S. Barnes Nov 24 '16 at 11:25
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Definitely plug wire(s). To confirm this take a look in the dark...enjoy the fireworks.

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