The reason heat shields gets stuck by carbon is they were not sealing/ seated properly. Clean all mating surfaces before installing new heat shield. A good way is wrap a wad of denim from jeans (tough cotton) around a flat blade screw driver with twist tie, dipped in penetrating fluid. Twist it back and forth in the pre chamber to pick up the gunk. Change denim as often as needed till no more gunk is on the denim/ pre chamber.

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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