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Re: Could I have a bad plug?
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 12 Feb 2004 09:39:06 Share Post by Email
In Reply to: Could I have a bad plug?, John Roberts, Wed, 11 Feb 2004 19:23:22
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The resistance in the plug is much, much less than air, so it isn't enough to cause the spark to jump wrong.

My guess is that the plug wires are worn, and that when you did the installation, a little bit of insulation finally gave way, giving the spark a place to jump to. So yes, it is related to replacing the plugs, but only because the wire got jostled.

The lower the air pressure, the easier it is for a spark to form. Or, the higher the air pressure, the higher a voltage you need for a spark to form. Normally, the plug fires on the compression stroke, so the air in the cylinder where the plug is is at high pressure. A large voltage needs to build up before the spark jumps in the cylinder.

If there is a flaw in the spark plug wire, there is a current path from the conductive wire inner core to the engine through the air. Since atmospheric pressure is much less than in the piston, the voltage needed to make a spark is much less. As the voltage builds up to spark, it reaches enough to jump through the flaw. So although the gap on the plug is much smaller than the 1/2 inch from the wire to the engine, it LOOKS like a bigger gap because it's under pressure. Normally the insulation on the wire makes the 'outside' path less attractive, but if there is a break in the insulation (from age and movement), it looks pretty good.

Make sure the plug gap is right, replace the wires, and you'll be fine.

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