I haven't tried this yet but I think this will keep the leaves out when the car is parked. I don't know how it would affect engine cooling though.

Leaves season should be here in a few weeks so I'll be able to test it.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked