Originally Posted by AusMBtech

Well I don't know what model your "other" car is but if it has a 601, 602, 603, 615, 616 or 617 then the small filter before the primer pump is the "pre filter". It is mentioned in the w124 and 201 service books, i'm sure it's in the others too. It should be replaced at the SAME TIME as the main fuel filter.

On your 210 (if it's not cdi) then there should be a pre-filter mounted in the main fuel filter housing. looking at the front of the engine there is a 10mm nut about 1" to the left of the main fuel filter (on the top) holding down a round plastic filter. That's the prefilter. It's not fitted to ALL 604, 605, 606 engines, but most will have it.

Make sure you replace the oring under the prefilter as well, the engine won't start if it leaks.

CDI's don't have prefilters.