I have the plugs out of the car and noticed one had two full length cracks that are black in the cracks. What would this do to my performance? Would this be from tightening them wrong or could it happen from driving?

 

Usually occures at installation. Should cause a miss. If you open the hood at night you should see sparking.

 

Yes this can cause the plug to loose spark and usually is due to overtightening them during install or when you are turning them angling the wrench to put lateral pressure on the plug leed.

Replace them and you should be good. Make sure to keep your socket vertical on the plug and don't pull too hard to one side.

 

I guess I will have to be more careful. Better not use any swivel sockets or anything thats puts more presure on one side. We will see if the car runs better once the new plugs are in.

 

I've cracked a few myself. Usually from over tightening or tightening at the wrong angle.

 

How do you know if you are overtightening? What is the torque spec?