I used to have an '81 240D w/ manual trans and the breather went from the valve cover to the intake manifold. It didnt have a hole on the cover. I'm betting that a previous owner swapped an air cleaner cover from a turbo model. I would plug the hole if you already have the breather tube going from the valve cover to the intake.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold