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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.

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Robert S. Barnes
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2 Answers2

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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.

Robert S. Barnes
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Pᴀᴜʟsᴛᴇʀ2
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Definitely plug wire(s). To confirm this take a look in the dark...enjoy the fireworks.

Raymond Zimmerman
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