Burned Oil can end up that shiny tary black color.

Carbon is likely trapping the Glow Plug Tip in the area with the Red Rectangle in the pic.

Repeated applications of the Chemicals mentioned in a previous Post would help to soften and lube the Carbon that is holding the tip in.

You also might want to pull the Injector and look at the tip. Some Brands of Glow Plugs have been known to have the tips swell up.

Or worn out Injector Nozzles can cause Fuel to spray on the Glow Plugs and burn and deform the tips.

Attached Thumbnails

   

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84 300D, 82 Volvo 244Gl Diesel