Auxilliary fuel tank

[1980sdga]

I have one of those auxilliary tanks in my trunk as well. Have no idea how it's plumbed or anything. I swear I heard a pump running when I drove it with the back seat out...

I believe those tanks were dealer installed because diesel wasn't as available as it is now. Would have a phenomenal range if you had it up to speed.

[Casey]

I have one of those auxilliary tanks in my trunk as well. Have no idea how it's plumbed or anything. I swear I heard a pump running when I drove it with the back seat out...

Mine just had a hose going through a hole cut into the bottom of the trunk merging in with the main fuel hose.  Then on the top there is a very thin hose that came up to a little hole to the upper left of the main gas cap.  There is no way you could have possibly filled this tank via that small hose, so I'm guessing that it just automatically overflowed up into the extra tank when you filled the main tank.  The top of the auxiliary tank and the level that you get the fuel to in the nozzle if you really fill it are about the same, with the vent hose coming out at a higher level, so this seems to make sense to me.  Nothing complicated about it, and your gas gauge would still more or less work as expected - it would just stay on full for a long time.

[Ilike300sd]

I never thought about the availability of diesel stations.  That makes perfect sense.  One could imagine Mercedes Execs of the time thinking about how U.S. distances could be much more spread out than Europe in terms of geography (closeness of towns to one another) and also that diesel was used more in Europe than the U.S. so their were simply more stations in Europe.  I wonder if it was even an option vs. someting they did automatically on the 5 cyl turbo engines (or maybe others as well).  100% speculation and not based on anything I've heard or read.  Just guessing.  Hopefully someone on here has first hand knowledge and can tell us we're full of it and tell us the real reason.  Would be interesting to know. 

I'll ask the mechanic tomorrow if I can.  Those guys should know I would think.  My tank has a filler cap on the upper right corner.  It's a rubber stopper similar to a drain plug on a boat but much larger.  There is also a little 90 degree tube on the top in the middle.  It's a poly tank that is pretty opaque natural/beige-ish color.

[Casey]

I'll ask the mechanic tomorrow if I can.  Those guys should know I would think.  My tank has a filler cap on the upper right corner.  It's a rubber stopper similar to a drain plug on a boat but much larger.  There is also a little 90 degree tube on the top in the middle.  It's a poly tank that is pretty opaque natural/beige-ish color.

Ahh that's very different - mine's steel with the same material that lines the trunk glued to it so that it blends right in with the trunk innards.  That vent hose I mentioned is connected to a 90 degree connector, but there is no other hole in the tank except for the big one at the bottom.

[1980sdga]

Here's my tank:



I believe it runs to the filler hose like yours Casey.  Hard to believe that is how it's supposed to be filled 

Did you pull yours out?

[Casey]

Here's my tank:

I believe it runs to the filler hose like yours Casey.  Hard to believe that is how it's supposed to be filled 

Did you pull yours out?

Oh, that looks just the same one I've got.  You just mention a filler cap on the top right - mine doesn't have that.  Is yours metal?  I thought you said it was polycarbonate...  I believe it just automatically fills when you fill the main tank, but I don't know as I never used it.

I pulled it from the junkyard 300SD and never got a chance to put it in.

[Casey]

Found online:

"It appears to be filled throught the stock filler neck, but I have a heck of a time figuring my fuel mileage since I can't seem to fill up the spare tank without slowly adding fuel and shaking the car to keep adding to the tank. From under the car I can see where the tanks are connected by what looks like a 2" radiator type hose. Anyone out there have this kind of setup? I don't know how else I can fill the spare tank short of taking it out and having a welder put a filler tube on it."

I would imagine it would be slow to fill or not fill at all if the vent hose is clogged at all, so check that and replace if necessary.  If you find any more information on this, please let me know.  I plan on installing it in my next 300SD, if I get one.    The capacity is 20 gallons bringing total fuel capacity to around 40 gallons, and it is claimed that the gauge continues to operate as expected - half full indicates 20 gallons of fuel remaining rather than 10.   I'm half-tempted to use it in the 450SE even though it's not safe for gasoline (because of the vent hose I'd guess) because I have to fuel up so much more often now, but wouldn't want to drill holes in the chassis for a temporary installation.

My W124 had a second filler cap that fit in beside the main one - it's cover was large enough to accomodate two of them.  But somebody had removed the tank before I bought it, so you had to be careful to make sure you filled your actual tank with diesel, rather than your trunk.

[1980sdga]

No, OP said he had a poly tank with a filler. I think I have the same deal as you.

If it has a large tube connecting the 2 tanks on the bottom maybe that's the way it fills and the top tube is a vent 

Mine is not filling the aux tank though. At least not much. I've never had to put more than 16 or so gallons in it and the fuel gauge seems to work.

I would like to get it operational. Be nice to get plenty of fuel when the price drops!

[Ilike300sd]

my plastic tank has the filler cap where that black box looking thing is on your photo (upper right).  It has a fitting similar to yours in the same spot in the middle.  Plastic though, not metal and no carpet. fyi

[Ilike300sd]

I got the ok from mechanic to use the extra tank as is.  I'll have to try it out soon...

[300SDude]

I have an extra fuel tank located in my trunk (although I wish I could have ran it across the back of the trunk) that is my WVO tank.



When I had it installed, my mechanics told me that I had to purge the system for 2.5 minutes whenever I turned it off.

It seemed to quit drawing from that tank when it was about half empty, so I took it back into my mechanics and had them check it out. They informed me to cut the purge time down to 1.5 minutes. Come to find-out, when purging the system, it was actually filling the WVO tank back up with diesel. As a matter of fact, it filled it so much that it was pushing it out through the cap and the sensor unit in the top of the tank.

Needless to say, I just quit purging the system all together and just be sure that I turn the WVO off a few miles before I am going to shut it off.

I am sure the purge system would be more needed in the winter to clear the lines. (The WVO has to be heated to 105 degrees or more before you can start using it). This is just my summer play car, so we don't drive it in the winter and I don't much worry about it. It seems to be running great now, and it is actually using the WVO like it is supposed to.

Someone mentioned that they could have sworn they heard a pump when the back seat was out, but I don't know if they need an electric pump to run the fuel from there. There are no electrical parts to the actual fuel system. The only thing that is electric is the light on the switch and my extra fuel gauge.

My Suburban does have an extra fuel pump mounted to the fender well under the hood to assist it using the veggie oil from the back of the truck. I have had to replace it once in the 4 years I have had it. And come to find-out, it was a defective pump and they pump place replaced it at no charge. (gotta love lifetime warranty ). I am wondering if you had an extra pump, it may make filling that extra tank a little easier. (Maybe mount it on the trunk floor and have a switch on it in the trunk to fill the inner tank before your main one). I have seen one other Mercedes that had the BIG gas door on it with 2 filler tubes as well. I thought it was some custom job or something.

The A/C in mine doesn't really blow cold, but I never use it anyway so I am not all that worried about fixing it, but the heater seems to only blow warm air when I use the defroster. When I put it on bi-level or anything, it doesn't seem to blow warm. Sometimes it seems like it is blowing cold/cool air instead of heat, so I make a fist and kinda punch the air control surround (not the buttons) and it starts to blow warm air. I am thinking that stupid spinny-wheel thing the US models got is the culprit. (Although I am sure the A/C motor has issues is why it doesn't blow cold). I haven't really checked it out.

I would LOVE to replace it with the manual controls like Squiggle Dog did.........how is that working, BTW?

[curbill]

We have a 42 gallon tank in our 1980 300sd. First time I realized how large it was when my wife was complaning that the gas gauge was not dropping. When it finally dropped down to 1/2 tank we stopped and put 21 gallons in it. Next time she said we needed fuel the gauge was down to 1/4 and I put 30 plus gallons in it. She had driven over 1,000 miles but negleted to mention that. As we have a 126 and a 116 now she just refers to the dark car or light car. When my daughter stops buy I want her to take pics of the setup and put them on line. Our installation is very professional and does not take extra time filling up.

[300SDude]

We have a 42 gallon tank in our 1980 300sd. First time I realized how large it was when my wife was complaning that the gas gauge was not dropping. When it finally dropped down to 1/2 tank we stopped and put 21 gallons in it. Next time she said we needed fuel the gauge was down to 1/4 and I put 30 plus gallons in it. She had driven over 1,000 miles but negleted to mention that. As we have a 126 and a 116 now she just refers to the dark car or light car. When my daughter stops buy I want her to take pics of the setup and put them on line. Our installation is very professional and does not take extra time filling up.

I am getting pretty close to 30mpg in my 116....don't know if this is good or bad??? I filled the tank at the beginning of the summer, and I still have half a tank (used 3/4 of a tank of veggie). I don't have to fill the 116 but MAYBE twice a summer for as much as we drive it at the moment.

We do take trips over to the coast to visit family every summer. (300 miles or so). I am hoping to figure-out and fix the light issues mine has (turn signals & back-up lights) before next summer and we'll take it on a road trip over there. That way I can really check what kind of mileage it is getting.

I love my 116......even if it wasn't running, I would just go and sit in it because I think it's cool, lol.

300SDude

[Ilike300sd]

I put 10 gallons in the auxially tank yesterday and didn't fill it up that much.  It looks like the tank could hold 25-30 gallons.  Along with the stock tank that would give quite a range.  There is a little ope/close valve on the back right corner of this tank and a hose going down to the stock tank.

[Ilike300sd]

Took a trip about 600 miles or so in the 300SD and the  extra gas tank really came in handy.  I turned the tiny valve on when the gas was getting low and it slowly filled the tank as I drove.  Pretty weird to see the fuel gauge getting fuller and fuller the farther you drive.