Okay, engine bay system has been eliminated... No bubbles this morning. Working back this evening to the short rear supply line.

I realize this is a slow moving narrative with a questionable pay-off. Hoping it's future-helpful.

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1981 240D 4sp manual. Ivory White.

You did test by clamping the line also just before the pre filter? Common area for a lot of grief. Normally but not always when an air leakage is back farther. You see a tiny stream of bubbles processing through the primary filter. When the engine is running.


Even if you see one tiny bubble in transit it is too much in your case.

Questionable payoff? Still don't trust the process? I am going to guess you have a bubble this morning after clamping the supply line under the tank.

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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