My first guess is a clogged injector. Try running Diesel Purge or some type of fuel injector cleaner (for diesels) through it. It could also be algea contamination.

There is also a fuel injection pump rack spring that might need replacement. It goes in the end of the pump and dampens the rack movement (internally inside the pump) Do a search on this site, you'll find it.

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)