Whatchuthinku?
Well as in proper Her Royal Majesty's English
fredbail I have news for you
Mercedes-Benz-Sprinter from the extensive listings of part numbers apart from to C clips between the Note:Plastic fuel tank to the rusty Neck Quote there are no metal fuel filler tubes
Behind the plastic fuel cap there are two of;and only two small sections of gold cadmium plated metal objects:Plated as a rust preventative of course.
IN the first and foremost line of reasoning to your whatdoyathinkum:... Rust is not present as such.
Fact your fuel and I hope it's ULSD summer grade therefore water in U.S fuels is present the rust is not rust but a rusticate ,thus is an algie bacterium that forms a fungus on the rear plated fuel cap and the metal plated locking mechanism the lockes your cap down [AIRTIGHT it's vacuum SEALED]THE AIR PRESSURE IS ATMOSPHERICALLY INERT.
"What to look for"
Open fuel door flap,remove fuel cap and look at the metal rear section behind the
plastic cap!If you can see water droplets then this is contaminated fuel from your supplier,or suppliers.
Over time the algie can attack this section and cause the rusticate to attack and form a rusticate fungus that eventually eats the metal out and real rust forms.
Basically the back metal section would fail and collapse,this has not happened to you unless your living in the Artic region of minus 30fC permanently.
I'm as usual prepared to back this up with H/D photographic evidence
The cap can not rust if an additive is used
And I can prove it
The long term preventative solution is to add to your fuel a Non alcohol based upper cylinder lubricant with anti-gel properties such as Red-line 85plus with winterized anti gel agents.
Red-line is guaranteed to work as I have used it full time since January 2005
There is no rust in the tank, fuel lines, or even a hint of injector damage or leakage anywhere.
If Alcohol based additives are used
{Be Warned} this will eat out and damage any copper component in your fuel lines to the injectors O rings.
I would strongly advise you to lift the engine cover and look for Black death before you even think of wasting $1200 dollars of your very hard earned money
Or go to a authorized Mercedes Out let for a proper diagnostic report.
What sediment that is collected at the bottom of your tank; if present may go to both Oil and fuel filters they will capture any large contaminates.
US Winter based fuels are your enemy not your friend Use an additive.
RICHARD