miricle grow of carbon on spark plug, cant get plug out

[el segundo]

On a F250B model year 2008 (V6 with right side head/center cylinder failure), the starboard center sparkplug and cylinder walls are completely coated with carbon. The motor was bought in 2013 and has 350 hours on it and just out of warranty.
The plugs went 200 hours (125 hours to 320 hours) without changing, when I tried to remover the starboard center spark plug I found it impossible to remove spark plug. It took 5 hours with impact wrenching and ring free/penetrating oil sprayed repeatedly drilling in and out repeatedly to knock down the large popsicle of carbon buildup of the spark plug tip. The diameter of the carbon clump was too big to get out of the cylinder head. What would cause this?
After 20 hours, I changed all spark plugs again. Again, it appears the problem is going on still. Does anybody have an idea of what the problem is?
My guess is there is no spark, BUT, without heat, how does the carbon build and deposit on the cylinder walls and plug?
If anybody has an idea on how to address the problem, I would appreciate the input. Again, the motor is 1 year out of warranty but obviously, the problem went undiagnosed for several years.

Matt

[panasonic]

I wonder if you don't have a bad valve stem seal letting oil into that cylinder. Try installing a new plug and remove after an hour of running and see what it looks like then. Does it seem to run OK after a clean plug is installed?

[TownsendsFJR1300]

After 20 hours, I changed all spark plugs again. Again, it appears the problem is going on still. Does anybody have an idea of what the problem is?
My guess is there is no spark, BUT, without heat, how does the carbon build and deposit on the cylinder walls and plug?
If anybody has an idea on how to address the problem, I would appreciate the input. Again, the motor is 1 year out of warranty but obviously, the problem went undiagnosed for several years.

Matt

Agreed with Panasonic. It wouldn't build up carbon if it wasn't firing. The plug carboned up so quickly confirms oil getting into that cylinder.

I'd also be leaning towards a valve seal leaking oil badly into the combustion chamber, then carboning up the engine/plug.

Unfortunately, as bad as it sounds, your rings are probably carboned up badly now as well..

There is a dealer ONLY de-carbon product that would likely help clean up the carbon/rings but the seal(s), need addressing. (IE a valve job)

BTW, your really lucky you didn't damage those head threads...