Mark, my success is also due to cleaning glow plug coils coated with castor oil, some that has thickened. I have and use simple green all the time. It is probably the best cleaner overall to own. It will clean oil off parts if its not too thick or dried. I appreciate the idea of using lemon juice. The next plugs I clean that won't glow, I will soak them in lemon juice. A PH chart with substances will give a list of things that will clean whatever you have dirty.
I'm fairly certain I'm not communicating very well or there is a lack of understanding of the chemistry occurring. Glowing when connected to a battery is a different action than glowing from a catalytic reaction. Connecting to a battery causes current to flow through the wire which in turn causes heat due to the power dissipation. If the glow plug doesn't glow when connected to the battery, the problem is the wire is broken. That the glow plug glows does not mean it will, in fact, run the engine reliably.
Once running, the ignition process is the catalytic chemical reaction of the oxygen, methanol and platinum interaction. The platinum "catches" a methanol molecule and aligns it such that an electron is freed. This free electron can then interreact with other molecules and support the oxidation of the entire mixture, it is the spark side of the fire triangle so to speak. As the element ages the surface binds with other elements, such as oxygen or nitrogen causing oxides and nitride to form. These new surface molecules change the surface charge and interrupt the catalytic reaction due to the new charge arrangement not "attracting" the free methanol molecules. The ignition stops. Take a close look at the wire when diagnosing engine problems and if the wire has a white hue to it, the element is done. The element should look shiny bright sliver.
This layer is generally what is responsible for the loss in performance of the glow plug. In order to restore operation of the element that layer would have to be removed. It may be possible to mechanically clean the layer using a scotch bright of other means. Etching is a potential method and the agent would need to target the oxide or nitride, depending on what has formed, each of which has a different binding potential and either acid or base would be required. The acid within the chlorinated cleaner is likely responsible for this if it is indeed happening.
A layer of choke might also be responsible, as in what would be from castor oil, but the high temperatures during operation would generally burn these off. Choke forms in relatively low temperatures as compared to the combustion temperatures, high enough to crack the hydrocarbon but not high enough to cause it to burn. It's likely that there is some fraction of the "dead" plugs will suffer this ailment and and will "repair" with a simple soaking. Choke can be identified fairly readily and most modelers using castor oil are familiar with it. It's the black chunky stuff.
Then there is the high likelihood of plugs which have been changed as a diagnostic which truly didn't need to be changed. As an observer and totally ignored nerd, I have seen at least 30%-40 of plugs swapped without good reason. I've gotten used to this and simply watch. There is likely a high percentage of discarded plugs in this category and with todays prices for plugs makes it worth it to pick them all up.
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