You may also want to check if the engine has sucked some glow plugs that the top of the piston and the comustion chamber is in good shape still. When you say eaten is the element gone or just won't heat. If it is gone then I would check how bad the damage is to the chamber. If it is not so bad you can clean it back to new with some lapping compound a cordless drill and some steel wool that is medium to fine grade. It will take the marks out of the chamber and the compound will polish it back up.If the chamber is not smooth and clean (meaning no dimples or marks from detonation) it can and most of the time will adversly affect engine performance. I do not think that your idle is related to it. The base idle will be more then likely a needle setting,idle setting,or air leak. Head shimming will not really affect the idle unless it is so far off there is way to much volume the head being lifted to high. So check those. If you need anything in more detail IM me and I will answe whatever you may need to know for what I have spoke of.