A few months ago one of the members said also did it without removing the intake manifold. But, he is the only one I have read to do so. (From what I have read.) This is one of the models of engines that more frequently has stuck or difficult to remove (hex heads breaking off) glow plugs. It is also written that one of the keys to removing a stuck glow plug without breaking it off is good alignement of the wrench to the glow plug. All of the indicates that your chances of removing the glow plugs with no problems are greatly increased with the manifold off (at least for the first time you pull them). Also if you needed to apply heat with a torch to the glow plug you could not do this with the manifold on. It is also recommended that you run the eingine up to operating temp before you try to remove the glow plugs. Do you want to squeeze your arm between the hot engine and the manifold?
After you re-install the glow plugs with some type of Never-seez; next time you could try it without removing the intake manifold and see if it is possible for you.
We had this skinney guy where I worked who could do jobs in tight places faster/more easily than the rest of us could. It also depends on your dexterity.

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84 300D, 82 Volvo 244Gl Diesel