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I have an old opel astra 2.0 8v. Thing is car idles rough and you can feel it misfiring very often, mostly at idle. In the dark you can see 3 of 4 cylinders have the spark jumping off the wire boot and into the nearest piece of metal almost continuosly. I have two new sets of wires, i replaced plugs and coils. The problem doesnt change.

What can make spark jump beyond these?

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Really sounds like bad wires to me. The arcing means that there is an easier place for the spark to jump to ground than at the spark plug where it should be happening. Maybe the wires you are replacing with are defective?

Some possible reasons for arcing:

- A plug gap that is too large. If the spark can't jump the gap in the plug, it will find another path to ground. Although I think the gap would have to be really large.

- Bad wires. Electricity will find the shortest path to ground so if the wire insulation is bad it will exit before it gets to the spark plug.

- Weak coils Sometimes this will cause an arc but usually difficult to see.

- Defective or broken plugs

I am assuming you have a spark exiting from the plug boot. Have you used dielectric grease inside the boots? Also, you say there is an arc with 3 out of 4 cylinders. Is the non-arcing wire always the same cylinder? If so I would try to move that wire to a different cylinder and see if it is still working properly. If so, it's almost certainly the wires.

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