#6 plug wire fails periodically

My #6 plug threads failed (plug popped out) in December '06. I repaired it with a thread repair kit (tap to larger size hole, and screw in a sleeve that accepts the plug) and it has run fine since, but the #6 wire has failed twice since that time (causing misfire code P0306). The only thing I know of that is not ideal about my fix is that I did not screw the replacement sleeve down far enough -- i.e. the "lip" of the plug boot sits about 1/4" above the plug access hole instead of flush on it like the other plugs.

Could this somehow cause the #6 wire to fail periodically? I keep an eye on that plug hole to make sure dirt doesn't get in, and splashed-up water getting in seems unlikely but possible. Maybe condensation is entering thru the gap and somehow causing the wire to fail. Another thought I had is that maybe the sleeve does not provide a good electrical connection to the block, causing the electricity to seek other paths to ground - such as thru the wire, causing damage (this would be a horrible design flaw of the thread fix kit). I am hesitant to mess with the thread fix, but maybe I can screw it down a bit further to close the gap (if this is the problem).

Has anyone experienced this before? I searched and could not find a similar situation of periodic plug wire failure.

Any ideas or opinions would be appreciated.

What brand plug wires are you using? Some have had issues with off brand wires.

I would be curious if your plug is grounding properly through the thread repair kit. I guess the best thing you could do is if you have a multimeter with a long probe, take out the plug and put one end of the multimeter to the negative on the battery or the head or the block or anywhere else, then the other to the thread repair kit. Check for resistance, there shouldnt be a lot.
Did you use any type of thread sealing compound, like thread tape?
Was the thread repair kit designed for spark plugs, or just an off the wall that matched the correct pitch.
Also I would inspect your plug to see if there had been any metal that was left in the engine that might have caused the plug to short to ground.

also one more thing, is your wire actually failing? Did you test resistance through the wire, or are you just shooting a misfire code and getting a random misfire every now and then, because I get the same problem on my 98, although mine seems to get my #2 cylinder most of the time and sometime #4. According to the TSB mentioned in an earlier thread though it appears to be an issue with calibration of my ECU, at least I am hoping thats the fix for me, cause I have already tried 2 new sets of plugs, currently running iridium plugs, and I replaced a set of wires despite the resistance still being within acceptable range.

If the thread reapir kit is not screwed all the way down then the tip of the spark plug must be set back in the slot by the amount the thread is sticking up. This could be causing some problems. As above, are you sure its the wires that are failing?

Tony2005 said:

What brand plug wires are you using? Some have had issues with off brand wires.

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Autolite's premium brand----I believe it's called Autolite Professional.

FunInNorthDakota said:

Did you use any type of thread sealing compound, like thread tape? Was the thread repair kit designed for spark plugs, or just an off the wall that matched the correct pitch. Also I would inspect your plug to see if there had been any metal that was left in the engine that might have caused the plug to short to ground.

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I used no thread sealing compound, but the kit (made for plugs) said to use silicone to seal the sleeve in (which I forgot to do). I am glad I forgot because it seems silicone could increase resistance from the sleeve to the block. About metal in the engine, I was really careful and spent a lot of time vacuuming, using tape on the end of an antenna, and blowing out the shavings so I believe there's none but will check when I remove the plug.

FunInNorthDakota said:

also one more thing, is your wire actually failing? Did you test resistance through the wire, or are you just shooting a misfire code and getting a random misfire every now and then..

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It is not random misfires, the codes are always P0306 and replacing only that wire makes the car go from barely driveable to normal. About the wire, it seems the wire's insulation is failing, but not it's resistance (each time I checked resistance and it was in spec). Each failure I can see the spark jumping out of the boot/wire junction (when engine is revved). Whether this is caused by the plug's ground degrading (causing the electricity to seek the next best path to ground) or actual insulation failure at that point I am not sure.

I had a thought that maybe the sleeve/block electrical junction starts to degrade (possibly because of dissimilar metals) which causes the plug to lose its ground, thus making the boot a better path.

SVTC1136 said:

If the thread reapir kit is not screwed all the way down then the tip of the spark plug must be set back in the slot by the amount the thread is sticking up. This could be causing some problems. As above, are you sure its the wires that are failing?

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I am sure the wire is what fails, but can the plug being set back be the cause of the wire dying?

I plan on taking a look at the plug and in the cylinder this weekend and measuring the sleeve to block resistance.

Your high resistance at the plug area sounds correct.

It may be lame but I'd try putting a big soft washer/connector around the base of the plug, connected by wire the an engine ground.

Mike

mrutkaus said:

Your high resistance at the plug area sounds correct.

It may be lame but I'd try putting a big soft washer/connector around the base of the plug, connected by wire the an engine ground.

Mike

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Lame is right!! That has to be the dumbest thing I've read in a while. Not only would it be extremly hard to do it could cause the plug to be too loose and or not far enough in the head causing more problems.

Bad Idea :nonono:

BTW how old are the wires? and you said it has failed twice. Did you get a new replacement or put a used one in?

Plugs had copper washers for 50 years...

ctmystique said:

... BTW how old are the wires? and you said it has failed twice. Did you get a new replacement or put a used one in?

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Short Answer --> The wires were new both times and failed after about six months both times.

Details --> I replaced the wires with new ones (Ford Racing) when I did the thread fix (Jan '07). After the #6 failed (Summer '07), I put in all new Autolite Professional wires. When it failed again (two weeks ago or so) I replaced only the #6 with an old wire lying around my garage.

ctmystique said:

... Not only would it be extremly hard to do it could cause the plug to be too loose and or not far enough in the head causing more problems...

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I know my #6 (thread repaired) plug is not far enough in the head (see my other posts above). What problems does this cause? Could this cause the wire to fail somehow?

Unless it's just a coincidence, I think the diagnosis of too much resistance because of the helicoil thingy is valid. As you said, the voltage wants to get to ground the easiest way possible; if the resistance across the plug gap is too much (perhaps because of the helicoil-head relationship) it will go somewhere else and burn the wire.

Unless you want to redo the whole thread thing (and honestly I have no idea why it would be higher resistance; any grease or whatever between the helicoil and the head would be pressed out of existence by the force of the helicoil against the head...but it's a strange world),
why not try making a copper plug gasket with a wire soldered to it and grounded to a block bolt? Then see if it runs ok and if it burns another wire? What else to do?

Well, I'll answer my own question. If you don't want to try this, further insulate the plug wire with something substantial, such as a long section of fuel hose or something like that. (too radical?)

Being new to this contour "mystique" (8 mo., but with 48 yrs of automotive experience) I suspect all these complaints of plug wire problems are because of the high plug gap recommendation in the first place; when you have such a big gap it encourages paths to ground anywhere possible along the plug wire.

Mike

Mike