The whole point of priming is to fill the filter canister up and then the lines.

That nut on top of the secondary filter has to be down tight as you need to seal it COMPLETELY before priming has any effect. You're pumping your fuel/air right out through the leak you introduce by having the filter partially unfastened.

That's why you have to re-prime after changing that filter. The filter is part of what must BE primed under normal conditions. And to prime it, it has to be airtight.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL