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When I'm using a known good spark plug to test spark at each wire, do I need to ground the threads of the plug to the engine block like the guy in this video is doing?

What happens if I don't ground the threads and just let it jump through the air to the block? I'm asking cause that's what I did once; I could see an ark going from roughly the threads of the plug to the engine block and then also the regular spark at the end of the plug.

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Yes you should ground the spark plug. In your case the spark plug was close enough to the block to still jump the gap. If the plug was far enough away from metal nothing would have happened.

On a separate note.

When testing for spark it is recommended to use a spark tester. Using a spark plug is a poor test of the ignition system. In free air the spark plug gap poses a very small load to the coil. The spark plug loads the coil when it is in a compressed mixture of air and fuel. The spark tester is either rated for a specific Kv or is adjustable to different Kv. By adjusting the tester to the Kv rating of the coil and then watching the quality of spark the health of the coil can be assessed. The spark should be bright and blue not dim and orange.

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  • What if it's dim and blue / grey?
    – Robert S. Barnes
    Commented Mar 4, 2016 at 12:59
  • @RobertS.Barnes That is kind of a grey area (no pun intended). Some coils are naturally weak or poorly made in the first place.
    – vini_i
    Commented Mar 4, 2016 at 13:22
  • I mean on for example a four cylinder where three are bright and one is kind of so-so. It's sparking, but looks weaker than the others. Would that point to the coil or wire?
    – Robert S. Barnes
    Commented Mar 4, 2016 at 14:05
  • @RobertS.Barnes This is where you play musical component. Swap the wires, does the weak spark stay where it was or does it follow the wire? Swap coils, does the weak spark stay where it's at or does it follow the coil. Do a careful visual inspection. A carbon tracked coil tower would make the coil and wire bad at the same time.
    – vini_i
    Commented Mar 4, 2016 at 15:01
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    It is worth noting, on some vehicles you can cause issues with the ignition system by not grounding the spark plug. If you allow nowhere for the spark to travel, it can fry the ignition components. Without knowing exactly if your vehicle is one of those, it's always a good choice to ensure the spark has some place to travel.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Mar 4, 2016 at 21:31

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