I was selling off the third row seat from my wagon and spotted a tank looking thing next to the spare tire, with lines going to the main tank. Is this an aux fuel tank or something more mundane?

I'll try to get a pic of it later.

What car are we talking about?

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman

83 TD

I believe W123-W124 wagons have an expansion tank like that, probably because of the funky pancake design of the main tank.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

Yup, it's the expansion tank.

__________________
Chad
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys

How does it fill up? It seems to be above the filler neck.

It's only for fuel expansion. Like if you filled your tank in the middle of winter and then parked your car in a heated garage over night. The fuel expands as it gets warmer and travels into the expansion tank if needed. If it expands further than the capacity of the expansion tank I believe it would come out the vent near the rear passenger wheel. The vent looks like a little funnel at the end of a line.

__________________
Chad
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys