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Premium Member 2005 SLK55 AMG; 2005 SLK350
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My recent project R171 had fuel tank problems - a huge amount of corrosion so bad that I replaced it. I've always had questions about the internals of the tank so I took a few minutes and opened the old one up for a look.

There is no electrical wiring integral to/welded into the tank. When you remove the fuel pump and sender unit, all the wiring is removed. (I've heard rumors here that there was some kind of wiring built into the tank, but that did not appear to be the case, at least on this one).

The ventilation system is simple: two plastic roll-over valves in the head of the tank, one on either side, with air hoses leading to a junction in the middle; from there one line to the vent nipple, with no check valve anywhere in the arrangement. This is very puzzling to me because when I bought the replacement tank I could clearly hear and feel the tank "burping" as if it had one. (Maybe my old and new tanks weren't identical? The part numbers matched). No check valve implies to me that the tank is open to the atmosphere normally, which seems unlikely.

The steel filler neck extends several inches into the tank and has a plastic sleeve attached at the bottom. This sleeve has a float mechanism in it; a butterfly valve moves with the float so that as the fuel level rises above the sleeve, the butterfly valve is closed. I guess this is how it shuts off the flow of fuel when refueling.

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