Regarding blocking the air, I've considered adding a system to force the "pressure control flap" shut on my 2.5L when the key is turned off. I have a spare vacuum solenoid that used to run the EGR valve available, or maybe the existing vacuum shutoff line could be tapped into? Though I'm hesitant to modify the existing shutoff system.

While learning about the EGR I found that if the engine is running at anything higher than idle, manually closing the flap will basically stall it.

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1993 300D 2.5L Turbo
1991 300E, looking for diesel to swap in