Glow plug seal not seated properly allowing carbon past the seat could be from:

1. glow plug not torqued properly
2. glow plug seat surface not clean
3. damaged glow plug seat

The glow plug seat seal can be tested by filling the outside of the glow plug where it threads into the head with shaving cream and cranking the engine a few turns while watching for bubbles. If you see bubbles in the shaving cream, the glow plug seat is not sealing.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked