First thing I guess at would be a loose connection somewhere between the
tank and the IP that allows air to enter the fuel system.
Loose hose clamps,old cracked plastic(by now they look brownish/yellow)fuel
lines (on the MB),old cracked rubber lines.The VW MAY have a fuel/water
separator (underneath the passenger side chassis,just forward of the rear
wheel...funny looking square box with a knurled plastic wheel you'd turn to
bleed off the H2O...Notorious for breaking off...might allow air in?)mine did.
The lift pump on the MB is always a favorite location.

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'84 300SD sold
124.128