2005 320CDI Fuel Cap

[Woo320]

I noticed that every time i refill the tank, the check engine light does not come on. But when my wife refills the tank, almost 3 out of 5 times the check engine light comes on.
At first, i thought about buying a new fuel cap, but a friend told me that the fuel cap must be from MB or else the check engine light will always be on.
My dealer tells me if the problem persists they will replace the fuel cap(free). Anyone out there encountered this problem.

[Bela]

I can think of two possible causes, although I would think dealer probably mentioned them: (1) the cap is not tightened completely, which WILL cause the warning light to go on; or (2) the tank is over-filled / topped-off, so that there is no breathing room at the top for the fumes to be sucked off, triggering the light because the vapor recovery system is unable to function. (These are my guess based on how I imagine these systems function; I may be wrong. Vapor recovery may be a little different on a diesel.)

The first is like having a bad gasket causing a lack of seal between the filler neck and the cap, which is why the dealer says a cap replacement might be necessary. However, if only one of you has the problem, it sounds like the cap is OK, and it's pilot error.

Bela

[WOOCHOW]

Is it possible she is leaving the engine running while re-fueling? Many times that will set off a CEL.