hello and welcome to justanswer.com
there are only a couple of things that can cause this but i would tend to agree with you .
try removing the plugs and checking them with a feeler guage as bad install can cause them to close up and them not notice .
hope this helps
the coil pack have diodes inside it which only allow the electrical impulses one way normally unless one of these diodes is damaged.
bad handleing will problebly have caused your problem.
and it would cause arking
also if they caught the leads this could also cause the arking .
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