Congrats! looks like it's an air leak problem after all! Try tightening the lift pump fittings and primer pump. Did you have the correct seals when replacing the fuel filter? Make sure those are clean and tight.

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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