How do sparkplugs *quit*?

[tyman]

hey. sounds dumb.. but what happens to a sparkplug when it quits??? i know WHAT causes it.. just not WHAT happens to/inside the plug??? anyone wanna explain please? and is their any way to revive a *dead* plug?

[4cylinders]

hey, sparkplugs can foul, or break down inside. they can also become physically damaged beyond repair. fouling can be cleaned off sometimes, not always, metal contamination can't. over heated pistons will transfer alumnium to the plug ceramic, and they will by-pass the gap. If the resister in some plugs gets damaged, the plug will open up, no current flow. sparkplugs are considered a consumable, or throw away. so buy them as cheap as you can, keep spares, throw the old ones away. also consider that some brands may work better for your particular application, so try different plugs.

[TimSr]

When you look at the bottom of a plug, The center electrode has a white ceramic insulator that separates it from the ground wire (the "L"). When voltage is stepped up high enough, that the current actually jumps the gap, creating your spark. Current alwasy travels the path of least resistance.

About 99% of the time, what happens is that the ceramic insulator around the center electrode gets coated with crap that is semiconductor in nature. The current begins to travel thorugh the crap coating across the insulator instead of jumping the gap. This is why you cannot clean a plug with a wire brush or anything else other than a sandblaster. You must get the insulator clean, and the conductive deposits off of it, and you cannot get in there with anything else I know of. It is not very common that something internal in the plug goes bad, other than something physically broken, such as a cracked insulator, or eventual corroding off of the "L". I have 15 year old plugs I am still running, that have been recleaned scores of times. A plug is pretty simple, and non-technical. It separates your coil output from ground by an insulator, and a gap small enough for your current to jump when stepped up to a high enough velocity (voltage).

[HEAD09]

just one more thing to add...even thou it may seam to work when you test it out of the cylinder head under pressure it may not work...working on planes before we stick it back into a plane cylinder we have too do a pressure test to make sure its okay...