Guessing that you might have a diesel (you didn't include a model) the primer pump is used with diesels to get fuel to the injector and to purge air in the system.

Haasman

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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)