Spark plug burnout

Too Hot or Lean Pre-Ignition Range. Pre-ignition or Arching. Need new plug wires too and use dielectric grease inside the boots on both ends. Also auto-ignition (i.e. knocking Pinging, too low or too high octane), run hot, low octane gas, high octane gas may have been added to which makes hotter temperature inside the cylinder. Check computer for codes knocking or may have the wrong plugs used, after-market plugs ect. Only use the OEM plugs gapped correctly. Gap may be off. May be out of time ever so slightly too, may mean bad timing set also, or bad sensor or computer.

Wrong gap or knock due to the 87 octane fuel.

Or you are suppose to use 87 octane and your using hotter fuel. High octane or even maybe a lean mixture problem. Running too lean.

Common problem with Kia. Detonation most likely with the knock sensor not doing it's job. Running a higher octane fuel such as premium may help. Also check with Kia to see if an updated ECU program is available for your vehicle.

Similar problem with different model at the link. http://deadkia.com/?page_id=14