I'm new to the board...but see lots of good information. Just a quick question for you knowledgeable types. I have a 2001 4.2L with about 67K miles. Recently, it started to miss during acceleration...and got worse with time. Finally, the SES light came on. Took it to my mechanic. Turns out the fail code showed a miss and the guy finds that two plugs were cracked (on the insulator, which he showed me when I got to the shop). As a result, the spark was arcing across the plug (from the crack) to the tip and burned up two plug wires. Replaced the coil while we at it.

Is it common for plugs to crack? I'm not a big time mechanic, but have changed plugs enough to know I haven't seen that! Without a mechanical breakdown, I'm guessing that they may have been cracked in the installation process. Any thoughts?

Thanks much!

 

If the engine is running right after the plugs where changed, then thats fine.
Trying to guess, after the fact, is of limited value, what happened to the plugs.
If they lasted many thousands of miles, you got your moneys worth before they broke.
It is easy to break a plug using the wrong socket without a rubber center and pulling off center or slipping off while tightening.
Plugs can also crack from the porclian being under strain when they were made and finally give up with all the heat cycling they go through.
It's all about quality of workmanship , mfg, or installation.
What happens in the engine , you can't do anything about.

 

It happens...but its not very common.