If it is assumed the Glow Plug System is working correctly most people simply go through the Glow Plug Cycle and repeat the cycles as many times as is needed. To aid starting the Engine.

But, that taxes an old Battery and is worse if the Battery Connectors and Ground Strap (Battery and Engine) have poor connections.

Some people have found thier Engine started easeir after the Reamed or otherwise cleaned out the Carbon From the Glow Plug Holes.

If your Injectors are not atomizing the Fuel well at cranking speeds (low cranking speed is yet another issue) you Engine will be harder to start.

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84 300D, 82 Volvo 244Gl Diesel