Can a engine run ssmoothly with a broke in half spark plug(where the porcelain meets the metal)no vibrations & would the plug wire boot hold the 2 pieces together?
A sparkplug can work properly for as long as it will with a split porcelain. The problems that will arise is that there is now a place for electrical leakage (where electricity can find a way to ground easier). It will not take long for electricity to find it's way through it and to the ground. Save yourself some headache and just replace the sparkplug, even if it's a new one. The boot may or may not hold it together, but that is not it's purpose. The boot is there to protect the connection of the conductor to the sparkplug and prevent a way to ground for it. Once enough pressure gets put upon the porcelain part of the spark plug, it will crack further and ultimately fall apart. If you need the vehicle to run you down to the parts store and back to get the sparkplug replacement, you might be fine, but I wouldn't trust it even that far.