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Originally Posted by UberRoo
It seems to be the coil pack. The insulation on the two low-tension leads is cracked where they enter the coil. The coil was still sparking consistently, but it didn't always seem to ignite the contents of the cylinder. The arc in my inline spark tester would change color slightly when it misfired, but it never stopped arcing. I figured an improper fuel mixture and resultant misfire would change the impedance and thus, explain the slight change in spark color. I guess not. I'm not sure why a little bit of deterioration of the low-tension wire insulation would have any effect on the performance of the coil, but I also can' t figure out where the opposite pole of the high tension circuit is. It doesn't seem to be the bolt that attaches to the head, so perhaps it gets its ground from one of the low-tension leads. ??? Very strange.
Incidentally, screwing around with the fuel injector plugs produces an error code in the ECU, but it doesn't trip the trouble light.
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The low tension wiring should not affect the miss, as it still sparks when the miss happens. The inline spark tester doesn't prove that the spark is jumping the plug gap, only that the coil is producing the voltage. The current could be flowing to ground through the plug insulator. The only places the voltage can short out is,
1. In the coil itself, shorting across to the low tension wiring.
2. Down the side of the rubber boot over the plug.
3. Down the insulator in the plug.
The rubber boot would be the prime cause of shorts, the rubber ages to cause tracts of carbon to form down the side, these are a high resistance at first, to cause the occasional miss, becoming lower in resistance as time goes on to become the lowest path to ground.
The Hi-tension circuit is from the secondary winding, across the plug gap to ground, up the battery earth lead to the positive side, back to the secondary winding through the positive supply to the coil. This is electron flow that is the oppersit direction to the rest of the wiring circuit, to allow the electron flow from the center electrode of the plug to ground. This is the same as the older points ign system.
As for the cause of the miss ???????????? You have swaped everything other than the coil itself. Don't forget that the resistance at the 0.040" plug gap is a lot higher in the engine under compression, than it is out in the atmosphere. The spark should be able to jump at least, 0.750" at atmospheric pressure. Just be carefull the coil energy is very high, can cause heart to stop.
All that said mate, I don't think it would be the ignition breaking down, if it does it at idle.
sorry.
Harvey.
