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New acquisition 1979 300SD with auxiliary fuel tank

Started by Mergyver, 17 June 2017, 07:43 AM

Mergyver

Just picked up a 300SD in amazing shape. The car has an auxiliary fuel tank behind the stock tank. It is made of welded aluminum and mounts to the back of the rear wheel wells. The filler neck is accessed from the trunk. The tank is hidden behind what appears to be a stock piece of trunk carpeting fitted to the tank.

The tank is plumbed into the fuel system through an electric valve controlled by a switch mounted in the glove box.

I bought it from the second owner who was a friend of the first. He made it sound like it was a dealer installed item.

He also said the original owner would drive it from NYC to Palm Beach Fla with out filling up. Could that be true? It appears to hold at least 15-20 gallons.

Does anyone have any background on these tanks? Were they dealer innstalled? How should they be managed? There is no gauge on it so do you just run it dry and then switch over to the stock tank? I am worried that by running it dry you introduce air into the fuel system while at speed which could do damage.

Thanks
Bill
1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased

UTn_boy

To my knowledge there was never an auxiliary talk offered by Mercedes, and there is no option code for it or any evidence of it's existence in the factory parts books or EPC.  The only additional tank offered was a Jerry can that would fit either inside of the spare tire or in one of the lower left or right trunk wells. 

In reality, Mercedes would not have offered something like this without the ability to monitor the fuel level. Nor would they have offered it in aluminum.  And you're right to a degree about running it dry. It wouldn't cause any damage, but one would have to physically get out of the car to operate the hand primer on the side of the injection pump to rid the system of air.  Again, not a design Mercedes would have concocted. 

The tank may have been installed at a dealership, but that would be as far as anything related to the dealership goes.  For some reason people like to think that because the work was done at the dealer that automatically makes it a dealer item. 

If the set up works, then use it.  If it bothers you, then remove the additional tank and lines.

The average fuel mileage for the 300SD was between 25-30 miles to the gallon.  Sometimes less if the engine was not taken care of. The distance between NYC and Palm Beach is around 1,300 miles.  Assuming that the two tanks are the same size, we know that the range on a full tank of diesel was probably around 400 miles.  So at the most one could only travel 800 miles before having to fill both tanks up.  Though, if the aftermarket tank was larger, then that would skew the numbers and could possibly allow for one to make the trip without having to stop.  Hard to say though....it would be really pushing it. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

Mergyver

That all makes sense. Thanks.

I have the aux tank out of the car to clean it out. I was surprised out how well it was designed to make use of the space and how it was mounted so that it did not rest on the floor of the trunk. After all these years there wasn't even any rust around the mounting points or where the fuel line passes through the body. Wherever it came from it was well done.

I am glad to hear running it dry won't damage the injector pump or injectors. Based on that I am thinking I have to top both tanks up and see how far I can get befor it runs dry.

I took some measurements on the outside of the tank and roughly calculated a volume of 6,155 cubic inches. I say roughly because it does not take into account the thickness of the aluminum or various clearances formed into the tank to allow it to fit between wheel wells and raising the trunk floor to get access to the spare. That equates to 26 gallons. The owners manual says the stock tank has 21.7 gallons for a total of 47.7 gallons.

So at 25 mpg I should get 1,192 miles, at 30mpg I should get 1,431 miles. Wow, that is almost half way across the country!

So I need to start prepping the car to get as high an mpg as possible.

1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased

Squiggle Dog

I thought that the auxiliary tanks always had a sending unit in them that was wired to the original tank in series or something so that you could read the level of both. It is pretty cool to be able to go such a distance without filling up, but it's pretty hard to drive that far without rest breaks, so you might as well stop and fill up. Plus that extra tank takes up valuable trunk space. However, if you want to run on biodiesel, that could come in handy because there are not many biodiesel stations around. I like to run B99 as often as possible, but when I travel out of state, there is usually nowhere to get it, so I just run on regular diesel.
Stop paying for animal cruelty and slaughter. Go vegan! [url="https://challenge22.com/"]https://challenge22.com/[/url]

1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 350,000+

Mergyver

Hi Squiggle dog.

I agree, my bladder won't last 1400 miles but I have to go for it just for bragging rites.

This tank is plumbed for venting but there is no electric connection so no sending unit. The trunk still has plenty of room for my needs.

What is b99. Do you need heaters or special filters to run that fuel?
1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased

Squiggle Dog

I tend to get sleepy if I'm driving for too long without breaks. I used to drive a 1968 W110 200D from Washington to Utah in the summer to visit family and I think I could go maybe 800 miles on a tank (I didn't have to fill up until I was in Idaho) and then I would fill up quick and drive the rest of the 14 hour trip. By the time I got to Utah, I'd be so tired and out of it from taking no breaks.

B99 is 99% biodiesel and 1% diesel. They put that tiny bit of diesel in so they can still charge road tax on diesel. It's basically waste vegetable oil that's been highly refined so it doesn't have all the contaminants and coagulants in it. It's clean burning and sometimes if you're lucky it's less expensive than diesel. It has better lubricity than diesel and keeps your engine clean and with less carbon deposits. You don't really need to do anything special with it, but installing new rubber hoses is a good idea as it can melt old ones. Most fuel hose today is pretty resistant to biodiesel, but I like to use plastic-lined hoses and installed a Viton/FKM O-ring in the tank strainer. If your fuel tank is dirty, biodiesel can clean out all the gunk and clog filters. I've not had a problem with it.

I found that it gels around 17 degrees F, so you'll want to mix it with regular diesel if you are in cold weather. Biodiesel is the BEST cleaner I have ever used for getting grease off of parts, but it will melt some types of rubber, so keeping it away from rubber parts is a good idea. Biodiesel is not to be confused with waste vegetable oil, which I have found to be very messy, requires heaters and special filters like you mentioned, and in my experience, it destroys injection pumps. My roommate has a 1991 W126 350SDL that has an auxiliary fuel tank for waste vegetable oil. The car had about 60,000 miles on waste vegetable oil and it ran rough. When we did a diesel purge, it cleaned out all the polymerized oil out of the injection pump. The pump was so worn out from lack of lubrication that the fuel was leaking into the crankcase to the point that the entire engine's crankcase filled up with diesel up into the valve cover and then hydrolocked the engine!
Stop paying for animal cruelty and slaughter. Go vegan! [url="https://challenge22.com/"]https://challenge22.com/[/url]

1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 350,000+

Mergyver

Thanks for the tips. Is B99 more of a Midwest product? I have not seen it in Nj.
1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased

Squiggle Dog

It's all over the US, but suppliers are sparse. There may not be one in your area. It's not uncommon to find many diesel stations with a label on the pump reading "This fuel may contain between 5 and 20 percent biomass"). Biodiesel can range anywhere between B5 and B100 with B100 being the highest concentration. In the Phoenix valley, there is currently only one biodiesel seller. They sell B60 and B99. There are also some diesel stations like Love's that can have between 5 and 20 percent biodiesel added. Right now the price for B99 in Phoenix has fluctuated between being cheaper and more expensive then regular diesel.

I used to live in Washington and there was only one supplier in the Tacoma area. It was a self-service station unlike the one in Phoenix where an attendant pumps it for you. It went from being cheaper than diesel to being twice as expensive. I think it went from $2 a gallon to about $6 a gallon before I stopped going there.

I used to drive a city bus in Park City, Utah, and the buses ran on B50. When I drove my 200D to Utah, I found zero suppliers between there and Washington. The ones that were supposed to be in Salt Lake City no longer sold it, and the only supplier left was in Park City at the bus pump.

There are sites like this to help you find it if you decide to use it. http://biodiesel.org/using-biodiesel/finding-biodiesel/retail-locations/retail-map It looks like there are 5 possible retailers in your area.
Stop paying for animal cruelty and slaughter. Go vegan! [url="https://challenge22.com/"]https://challenge22.com/[/url]

1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 350,000+

Mergyver

Thanks for the website. Unfortunately It looks like it is only available for home heating, not for over the road in nj.
1974 450SEL
1979 300SD
1989 560SL
2005 E320 CDI Recently deceased