When the Glow Plugs use a Glow Plug Reamer to ream the Carbon out of ht Glow Plug holes or us an alternative metnod to clean the Carbon out. There is several Threads on various ways to do that.
Before installing them put some Never-Seeze type compound on the Glow Plug threaded area.

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84 300D, 82 Volvo 244Gl Diesel