spark grounding through plug
 

I changed my plugs the other day while changing the fuel filter. I had a loss of power that I attributed to either loss fuel pressure at high rpm, or a miss. When I pulled the old plugs (gapped too much .045) the porcelain area that the spark plug boot covers looked as if the spark was grounding through the plug. There was a 1-2 cm region that was orange, and then what looked like lightning streaking upward towards the top of the plug. The lines (lightning) were dark gray. I can't seem to find literature that explains how to read the part of the plug that is outside the head.

What would cause this?

did you happen to have water in the surrounding areas?
don't laugh but i once washed my engine and never thought any of it. once i started driving like 3 hours later, then engine would not rev over 4k (sort of like your problem) and i couldn't figure this out for the longest time. then i went to check my plugs and there was water on the boots... pulled them all out and realized i had a bit of water in one of the plugs. the others looked like what you describe. i guess the heat had evaporated most of the water. but the issue had been there non-the-less. obviously had to replace the plugs. and wires.
just my $.02

This probably means your boots aren't tight enough (Miatas seem to eat plug wires), and you've had arcing. If the surface of the insulator isn't still smooth and shiny, replace them, as the glaze has been etched. The boots are well down in the head, and all the heat seems to degrade them fast.

Clean out the plug wells, replace the plugs and wires, and marvel at how much better it runs.

Thanks, I replaced the plugs and decreased the plug gap. Ran much better. Didn't replace the wires. I'll have to do that. What's that average life that people are getting from their wires?

No water that I'm aware of, but I haven't owned the car for that long. I will pull the new plugs once every other week for a while just to monitor whats going on.

After reading a few posts, I think I still might be gapped a little wide (.030). I think I'll bring it down to .025 to be safe.

Don't know how many miles were on the set when I got the car, but they still looked fairly new. I replaced them anyway (NGK), and it seemed to make it run smoother. Also, don't cheap out on wires, the no-name ones won't last.