Originally Posted by barry123400

Not that I own one of these engines. If I did I would consider filling with a solution like marvel mystery oil and allowing to soak for days. That might dissolve or soften enough carbon to ease the removal somewhat. A help if it even just softened the carbon to some degree. . . The soaking carbon has to soften before it dissolves. Only the penatration depth/rate of the solvent into the carbon is in question here.
Sure some of the junk will go by the rings so perhaps do the glow plugs when an oil change is due. There has to be a less painful way than breaking off plugs and pulling heads.
The only real problem is the soaking takes time. So is basically not a garage option.
Yet if you have access or own more than one car and do your own work what is the downside? It is proven that the marvel mystery oil dissolves carbon. Mind at a slow rate. The rate of softening again is an unknown. Or perhaps someone out there does have the rate/depth/time equation.
Some other safe chemical might even be quicker.. Soft dissolved carbon will not hurt the engine.
If you were bound and bent to change them. They operate at a nominal 12v. What is going to happen to the carbon if you push each plug to 18-24 volts? Hot enough to burn it off? Is the heat too intense for the alloy head if the plug does not burn out?
Just a thought but again it would take quite a bit of knowledge and perhaps an old cracked head to do experiments on before even really considering a higher voltage approach.
You could even get the estimated time of dissolution or softening of the carbon with safe chemicals and the old cracked head. . I know the marvel oil works to some extent.
A member some time ago stood an old injector in a shallow container of marvel oil . I believe that member reported seeing a stream of black leaving the injector area in a couple of days time. That had to be the carbon around the nozzle dissolving. What is important and the member did not deal with. Was the remaining carbon still as hard to remove or getting softer as well?
If it tends to soften the carbon overall and I suspect it might. Rather than just the surface that is in an advanced state of dissolution that could be one answer to this miserable situation.