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Old 05-09-2007, 03:53 AM   #1
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Default Plug wires fail - cause miss

A couple of days ago I felt a single sporadic miss in the ’95 sitting at a stop light. It was a random single thump so hard that it shook the car kinda’ like when you shift the transmission into gear from park. No check engine light. No codes stored. Popped the hood in the driveway and noticed some suspicious looking marks on the #6 plug lead. Arcing maybe? So, I carefully twisted and pulled off the spark plug boot on #6. I re-routed the wire away from any possible grounding point and away from the oxygen sensor lead and took the car for a drive. The periodic miss was still there. The NGK 3951 plugs and the Premium Belden wires had about 2 years and 20,000 miles on them, so I figured I would install a new set of plugs.

Bought a new set of NGKs today, put ‘em in with anti-seize on the threads and a new dollop of dielectric silicone in the boots to make sure they continued to come off easily. Took it for a test drive – started out smooth, then warmed up and started missing – worse than before.

So now I’m thinking…new plugs, relatively new wires, must be something else. Maybe my fuel pump is going bad. Check the fuel pressure – 42 [email protected], 52 psi with the vacuum line off the FPR. Pull back the covers on the positive battery cable ends – no corrosion. Cover with dielectric grease and re-assemble. Check the coils secondary resistance – 6.36, 6.12, 6.16 kohm. Check the coils primary resistance – 0.6, 0.6, 0.5 ohm. Unplug the MAF, runs rough – plug it in – runs better.

Check for codes. In addition to the MAF code, I find a history code for misfire on #6. That rings a bell. Could I have messed up the wire pulling it off to check in the first place? Check resistance on #6 cable – 6.5 kohm. Seems about right. Did I mix up the wire routing? Check against the little diagram – nope, they are all where they should be. To check resistance on #3 cable, I twist and pull the boot and….the metal clip stays on the plug and the rubber boot comes off breaking the conductor in the plug wire. Crud…now I need a new set of wires. Off to O’Reilly’* for a new set of wires. Install them and voila….the miss is gone. If that #3 clip had not broken off, and all the wires tested OK, I would probably still be trying to find the cause of the miss.

What did I learn? A good place to start when a problem gets worse is to ask…what did I do to make it worse? Removed and replaced the boot on the #6 wire. It probably broke internally too, even though it tested OK. What else did I learn? Even though I was very careful to use silicone grease on all the boots, and to twist them on the plug to make sure they were free before pulling, and to pull only on the boot; the wires were apparently damaged by simply removing and replacing them. Do the wires get brittle internally with age? Two years doesn’t seem like a long time to me for a set of “premium” wires. But the wires I installed two years ago had no metal heat shields like factory originals. The shields were long gone before I ever got the car. I am guessing that without those shields, exposure to heat may make the conductor brittle and likely to break.

Although I have replaced plugs many times re-using old wires with no problems, I don’t think I will continue that practice with the 3800 engine. In the future, I will replace plugs and wires together.

Hope this helps someone else with a similar problem.
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Old 05-09-2007, 08:09 AM   #2
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my new wires are sitting in my back seat... been there for about a month after i changed plugs, didnt notice anything until i twas raining on my way home this morning... not good.
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Old 05-09-2007, 08:42 AM   #3
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Bill... next time pull out the hose and spray the engine. Any issues like that wire will usually manifest themselves nearly instantly.
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Old 05-09-2007, 09:22 AM   #4
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Not a broken conductor inside a boot.

Water only helps you find shorts through the insulation to ground. Or you can carefully inspect the wire insulation for gray/black carbon scoring.
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Old 05-09-2007, 09:34 AM   #5
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Quote:
Originally Posted by willwren
Not a broken conductor inside a boot.

Water only helps you find shorts through the insulation to ground. Or you can carefully inspect the wire insulation for gray/black carbon scoring.
Read closer BW. . BB said he saw arching on the outside of the wire. That #3 boot caused him to replace and find the #6 issue because it ohmed good.

(Coffee is good stuff..I dunno how you get up so early)
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Old 05-09-2007, 11:13 AM   #6
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I still have the old wires. I will do a little testing today to see if I can determine whether the insulation or the conductor failed.
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Old 05-09-2007, 11:27 AM   #7
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bill buttermore, great read, I'm hoping one day I've have the knowledge to diagnose like you guys and gals do. I guess it'* just a matter of experience, trial and error and so on and so on. Oh ya and the tools and this forum help tremendously also.


Sorry for highjacking this thread.
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Old 05-09-2007, 11:55 AM   #8
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I had a bad miss that I had backfires in twos. I got no codes and replaced the ECM. Same problem. I tested the ICM and coils and all were fine. Changed plugs and wires and found two plugs gas fouled. I had two bad wires. Never overlook a simple solution. I'm guilty, I always think it something of a bigger problem!
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Old 05-09-2007, 11:59 AM   #9
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Bill - don't the Belden premium wires have a lifetime warranty where you could get free replacements?

Many wires, I'd assume Belden also, have a center conductor made from carbon. At the ends, they shove a wire in, bend it around the outside of the cable and crimp the connector on. It'* relatively flexible but any sharp bends will break the conductor. It also has a weak point at each end where the wire pushes in.

When you are doing your resistance checks, gently bend each end of the cable and see if the resistance changes.
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Old 05-09-2007, 12:34 PM   #10
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Hmmmm i have a situation on my buick like yours....at stop signs or red lights i noticed that the idle seems a little rough...as if the idle is too low or i have a miss....im pretty sure its a miss, im not sure how to go about it and find the problem.
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