I have had this TD since last Nov. and only now noticed a black plastic tank that sits behind the spare tire. There appears to be 3 cloth braided fuel hoses coming from the bottom of it. What is this? This can't be the "reserve" for the fuel tank can it? I can't see how it could be since it sits higher than the fuel tank it'self. Speaking of "reserve", if this isn't it, where exactly is it. I have always wondered, on every car they always list a fuel tank capacity and say that includes a reserve of xxxx.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

your 'mystery' tank is the expansion tank for the fuel tank. When you over fill the tank with cold fuel and it heats up, it will flow into this tank instead of onto the ground. Not all models have them.

The "reserve" is how much fuel is left when the low fuel light comes on.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Bill:

On the W124 the windshield washer tank is on the other side with the tool kit (passenger side) and is white plastic. It also has a cap, the expansion tank doesn't. Not all wagons have the expansion tank, have no idea at all why!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Bill:

No one is perfect (I get reminded of that all the time -- right now every time I look in the back yard and see a big blue 280 SE .....!)

Nice finny, by the way. Looks almost perfect, unlike Big Blue, who has a cheap paint job peeling off and smells of very dead gas!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

My TD has no overflow tank. And I've had numerous opportunities to remove my spare over the last couple of years, so I would have seen it.

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Dave
1995 C280
1999 Triumph Daytona 955I - my speed fix.
1982 300TD - Gone, but not forgotten.

How does fuel flow up? The filler neck sits lower than this "expansion" tank. Also, what would heat up the fuel?

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

rickjordan,

The fuel is stored at the filling station at some prescribed distance below the surface, where the temperature is a pretty constant value. When you buy fuel, and it is foaming as it comes out of the nozzle, it is at this storage temperature. Your car and tank, and within a few hours, the fuel you bought, will be at ambient temperature. In the summer this is likely to be quite a bit higher than the underground storage tank temperature at the filling station. That is how it is heated.

I believe that foaming feature of Diesel is the reason you have the "compensating tank" as it is called in my manual. I believe Diesel foams while you drive, as well as when you fill the tank. On the W115 and W123 series vehicles, there was an atmospheric vent on the fuel tank that used the compensating tank to avoid getting the foam dribbling out of the right rear corner of the car. On most models this vent is just under the right rear bumper corner. The vent is needed to prevent sucking a vacuum in the fuel tank as you use the fuel.

I doubt this feature continued as even Diesel fuel vapors are likely to be restricted by the EPA. I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)