tight plug gap required, why?

[RonQ]

'98 Plymouth Voyage, 2.4L, near 95,000 miles
engines misses with ODBII error: "Cylinder 3 missfire". Self help needed since dealer support with diagnostic gives no answer.
Changed coil, plugs, ODBII, tested water temp and MAP sensor with replacements. Nothing found. Since "misfire" symptom could be plug gap, I changed plug gap gadually down to .18. Engine runs ok for now since abt 20,000 miles. But as plug gap gruadually opens up every 8,000 miles, I gap it back down to .18 and engine is mostly happy. Factory sticker says plug gap should be .50. What's up?

 

[RonQ]

Thank you everone, for fine help.
I'll go for a 0.040-0.045 plug gap and check in 25,000 miles.
I'll be away for a few days...
CUL

 

[patprimmer]

I agree with Warpspeed

An open top aids drying. A blast of air when in doubt is always a good idea.

Maybe you can buy aftermarket leads with long boots without the seal. This would avoid the extorsion of you must buy overpriced original leads as no others fit nar nar nar.



Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[racedoc]

This is a common problem which is made worse by lean fuel mixtures and the resultant high firing voltages. This is a huge problem on heavy-duty SI engines. Platinum plugs are supposed to last approx. 25K miles in these applications (which is hopeful) but in many cases the secondary insulation won't make it that far.

 

[Metalguy]

That's why I recommended he gap a 0.040". If he was able to run that thing at .018"(!), it should be fine at .040" and the voltages will be lower.

 

[Tmoose]

"It appears that 0.018 just out-competes a 1/4 air gap for juice."
I think the 1/4 inch air gap was no longer an important part of the circuit. Carbon tracking is a surface phenomenon. You may have had a carbonaceous "jumper wire" running from the metal plug connector to who-knows-how-close to another metal surface. A loose fitting plug boot and a dirty plug body can be "bad plug wires."

I reclaimed several expensive phenolic plug extenders on a relatively rugged Peugeot 404 hemi by grinding out the jagged black carbon tracks. They optimistically included a 4 inch long steel spring as THE connection between the end of the plug and the plug lead, and expected the long phenolic tube to insulate the spring. That long sping made a wicked plug wrench necessary, but did make retrieving the plug from the deep dark hole much easier.

I figure carbon tracks are initiated by a spark crawling on a wet surface. Excellent Mechanical sealing of the boots are the answer. Silicon dielectric grease is part of excellent mechanical sealing.

"I have seen one wire set(for different car make) come with dielectric gel in a packet to line the inside of plug boot. Maybe this 2.4L Chrysler engine needs this/ could benefit from this gel too?"

I am real fond of using that grease to seal hi and low tension connections and thus preventing access to conductive water/oil film. Also to keep plug boots from sticking to plugs, and weatherstrip from freezing to metal doors.