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While looking for the reason of occasional misfires I checked if my spark plug wires are arcing at night. There was no textbook arcing with popping sounds and pretty lights, but there was extremely slight, barely visible arcing when wires are moved near the block (or even when I put my finger near them, goes straight to it). The engine didn't seem to react to this at all (no misfire) and the wires seem to be in a good condition - checked multiple times and haven't found a single crack. Spraying water on the leads makes no difference.

Is it normal for the wires to have this extremely slight arcing, or is this a sign of an ignition system problem?

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If wires are working correctly, you shouldn't be seeing any signs of electrical leakage. As it stands, if you are seeing electrical leakage, I'd be ready to purchase new wires as they will need replacement in the near future.

You can test some wires by doing a resistance check. Checker Auto states:

You can test some types of spark plug wires with an ohmmeter. Basically, there are three spark plug wire types, but more on that later. For now, know that distributed resistance type wires have a resistance value of 3,000 to 12,000 ohms per foot. MAG and fixed resistor spark plug wires have much lower resistance per foot. But the actual resistance per foot isn’t important. It’s the consistency of the readings between each wire that determines if the spark plug wire is good or bad. So, measure the length and resistance of each spark plug wire and write it down. Then compare the resistance values. A bad spark plug wire will have much more resistance than another wire of equal length. That’s your signal that it’s time to change spark plug wires.

There are (basically) three different types of wires:

  • Distributed resistance
  • Inductance (MAG)
  • Fixed resistor

You can only measure the resistance on the Distributed resistance type. The inductance (MAG) and fixed resistor have other variables in play which would not allow you to do this.

  • The resistance of wires is quite low, it was around 1.6kohm if I recall correctly, same for both wires. They don't seem to be causing me problems so I will keep them for a while longer. – I have no idea what I'm doing Nov 23 '15 at 9:21
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I have observed a glow on new spark plug wires. It is called CORONA. It will go to your fingers without shocking you. I would increase the wire diameter if it was wire change time and your application will accept the larger size wire, otherwise I would consider it normal. On a rainey humid night you should see no visable sparks. Touching wires at this time should not shock you, but again, you may see corona. If you are ever shocked, replace the wires ASAP.

  • Not exactly a glow, but tiny barely visible discharges that do not shock. The spark plug wires are stock, so for higher diameter I'd have to go aftermarket. And the unusual shape of the wires on older Volvos means that it won't be an easy task to find some. – I have no idea what I'm doing Feb 9 '16 at 8:04

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