Probably a plugged tank vent valve or plugged tank vent line.

See post # 13 on this thread.
http://www.peachparts.com/shopforum/diesel-discussion/212289-fuel-tank-vent.html

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022