I have no specific idea. When the crank damper goes bad it does not dampen vibrations correctly. So that is one possible.

This is generic, on some engines there is a cupped spring washer/s used with the crankshaft bolt that are a onetime use item.

Some crank bolts are onetime use items.

If the Sprinter engine is a mechanical fuel injection engine bad injector nozzles and uneven fuel injection can create unusual vibrations in the crank shaft as can uneven compression. What people don't realize is that when the fuel fires and the connecting rod starts down the crankshaft flexes before it moves. It needs to flex pretty much the same as each cylinder fires. That means events not happening correctly in the combustion chamber can affect crankcase vibration.

Also, once the bolt vibrates loose there is some chance the crank threads got buggered.

It is hard to give a definitive answer and paying someone that does not know how to trouble shoot the issue is going to be prohibitively expensive.

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84 300D, 82 Volvo 244Gl Diesel