Forget original equipment. The majority if not all cheap diesels have drainable water separaters. On the normal mercedes set up I believe the water accumulates in the secondary fuel filter till it spills over into the injection pump.

That is about the same or worse than sandpaper processing through the element bores. Can seriously damage the injector tips as well. Even known to tear metal in very close proximity to other metal. Such a situation exists in the element bores.

I almost have to believe mercedes felt their upscale patrons would either not know what one was or use it. Owning other diesels over the years. The non presence of a manual or automatic water separator in mercedes diesels really suprised me.

I install an aftermarket one if the car is in service or will be. It also has a light to warn me of the presence of water in the bowl. This to me is really important.

If you fuel up somewhere when traveling. The addition of a lot of water in the new fuel should show quickly enough to get back to the station before the engine quits. There should not be much discussion about them having the fuel removed or drained down far enough to eliminate the water. You have the proof.

Since the fuel oil sits on top of the water once the sloshing in the tank has stopped for a few minutes. Draining just some of the fuel out should get the majority of the water. You can visually see the water in the bottom of a bucket for example. It separates quickly.

Otherwise you are way up the road and the car starts acting bad or even quits and you are not sure of the cause. Suspicion will not usually get the station that sold the fuel to deal with it. Plus you otherwise may not recognize the cause of your problem.

I have had enough experience with fuel vendors introducing problems over the last twenty years to really watch where I fuel up. Not so much in the last five years. Could be they have improved as the diesel gallonage has gone up. More likely it is I watch where I buy from.