Fuel Tank Tour

[ja17]

*first photo updated to show fuel inlet passage to flower pot 11-40-6

The inner workings of the W113 fuel tank are a mystery to most owners. I had an old tank which had been open and dried out for years. I cut it open and took some photos for everyone to see. If you ever need to re-condition your tank or service the drain plug/fuel screen this information should be helpful and probably not available anywhere else.

Here is an illustration of the complex "flower pot assembly" (thanks Dan for the terminology).

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The drain plug/fuel screen in your fuel tank is the first line of defense to protect your injection system from contamination. This drain plug/fuel screen can be removed using a 22mm allen bolt (7/8") will work. I made a tool using a 22mm bolt welded on a pipe (see picture). The unit has a rubber o-ring seal which you may wish to replace with new. New drain plug/fuel screens are available new if yours is damaged. My old dealer (280-SL)parts book show the number for the filter/plug to be #111 470 06 86. The seal is listed as #110 997 01 45. These numbers may have changed. Any dealer should be able to cross ref. these numbers to current ones. Earlier W113 cars may be a little  different.  I believe the earliest versions of this drain plug/fuel screen may be metal the later units are plastic. In addition the early units may use a different metal seal instead of the rubber o-ring seal. Maybe some others can comment here.  

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The next photo shows the tank bottom without the flower pot to show how the fuel manifold interacts with the fuel pick up line. Fuel going to the engine must first flow through the screen and into the round "fuel manifold" where it leaves the fuel tank via the metal line. After traveling through the electric fuel pump, fuel lines, main fuel filter, injection pump, left over fuel is returned to the tank via the smaller metal fuel line and dumped into the "flower pot".

Notice where the fuel return line and the  larger (lower) fuel supply line enter the tank. Notice that the metal fuel supply originates at the at a round collector (manifold) in the center bottom of the tank where it picks up fuel after it is drawn through the drain plug/fuel screen.

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If you ever need to recondition your tank and coat it with gas tank sealer you should be very careful to cover the screen so sealer will not clog the round collector (manifold and fuel line.

Also be aware of the tiny vent line which snakes along the top of the tank! This must also be kept clear!



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This tank seems to have a zinc galvenized coating from the factory and the surface is still in decent condition. I am not sure that the earlier tanks were galvanized.

Use extreme caution when working around gasoline! Make sure to have enough empty containers to store the fuel in sealed containers. Have an extinguisher and something to absorb spills on hand. Static electric or things like water heater pilot lights can ignite gas fuems.  You may want to do this kind of work in the well-ventilated outdoors. You can  pump your tank dry by disconnecting a fuel line in the engine compartment hooking up a hose and use the electric fuel pump to pump the fuel into containers.
Just be careful or have a professional deal with fuel tanks.



Joe Alexander
Blacklick, Ohio

[Douglas]

Joe,

I was wondering if you can just clean out the filter in the tank instead of replacing it. Do you think that's generally sufficient?

Douglas Kim
New York, NY
280 SL #016220

[ja17]

Hello Doug,
Yes cleaning the screen is usually sufficient. Replacememnt is needed only when the screen is damaged. These screens only catch the larger particles of dirt.


Joe Alexander
Blacklick, Ohio

[Sphe]

This is an excellent post! I always wondered how that little screen worked...

 My question is this. When getting my tank sealed, is getting the pipes clogged really something to worry about like you said? And since my tank has already moved to the shop, how can I undo this mistake if it so happens?

[ja17]

Hello Sphe,
The sealer used in re-coating the tank may completely block off or restrict the metal supply line in the tank. You must close of the screen and it's openings with something like shrink wrap. Then install the shrink wraped screen in the tank before the sealer is poured in. This will prevent the sealer from entering the round collector and plugging the supply tube.
If your tank has already been coated you must make sure the metal fuel supply tube is clear. Use a wire or a long brush. A rifle cleaning kit has nice long brushes, various diameters for gun barrels and works nicely.  blow compressed air through the supply tube when finished. Happy motoring!

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Joe Alexander
Blacklick, Ohio

[Naj ✝︎]

Joe,
My understanding was that the return line dumps fuel in the 'flower pot' (as described by the good doctor)   .
On my 230sl the return line was longer and with bends and had to be desoldered  and removed to clear it out.
Have you got a picture of the flower pot?
naj

65 230SL
68 280SL

[BHap]

so - now that you have the tank "open" - where do the two vent lines go inside of the tank?  I have a 1970 US version 280SL with the evaporator tank inside the trunk....  I've always wondered where the two lines on either side of the filler neck go inside of the tank...  And the natural follow-up question - which one is vapor and which one is fuel return....  any "enlightenment" is appreciated....

[rwmastel]

Another excellent "tour", Joe!

Could you take a close up photo of the filter and show how it forces fuel to go through the filter (said otherwise, how the base fits into the tank) and releases fuel out of the base?  Knowing this makes it easier to understand how the plastic-wrapped filter element protects the fuel passages during tank resealing.

Regarding your comment on older fuel tank filters, I ordered one for my 230SL from Tom Hanson a couple years ago.  It's base was metal and the filter was plastic.  I don't recall if it came with a rubber gasket, but I did not order one and it has not leaked.

Finally, regarding the 230SL tank question, I recall looking inside mine and seeing more tubing that is shown here on Joe's 280SL tank.  They are different and purchasing a new 230SL tank will cost significantly more.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[ja17]

Hello Rod and Naj,
Sometimes it takes a few of us to solve all the mysteries. After reading your post I went on a quest for the "flower pot" tank. After rummaging through my scrap heap I turned one up (see photo).


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The principal is the same as the previous tank but more complexity. The return line is connected with a clamped rubber fuel hose to the "flower pot"! Return fuel is delivered in a small resivour "flower pot" right next to the fuel pick up.


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This makes reconditioning even more difficult. Sealer will tend to collect in the "flower pot" and close off the return line. During and after the sealing process both lines will need to be kept from becoming clogged. Compressed air blown through the return line shortly after sealing will help. A flexible wire or cable (speedometer cable core)will be needed to clear any sealer or obstructions in the cuved tube. It looks like the tank should be set upside down after sealing so too much sealer is not collected in the "flower pot".

The same round collector as seen in the previous post is nested in the center of the "flower pot" with the fuel screen. these tanks are vertually the same except the "flower pot" and extra plumbing has been added.

Here is a photo showing the different diameters of the fuel feed and fuel return line.


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If the rubber fuel hose deteriorates in the tank, I suspect all kind of strange things can happen.

Joe Alexander
Blacklick, Ohio

[ja17]

Hello again,
The latest research provides some additional information. A 1970 vintage tank yielded an internal "flower pot" assembly with all metal lines. The rubber line as seen in the inside of the earlier tank has changed to all metal internal lines.

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Bhap, I finally got some information for you on the vent line. This photo shows the line which runs from the filler and along the top of the gas tank. This line goes to the plastic expansion tank in some of the models and to the charcoal canister in others. The expansion tank, or charcoal tank has a return line back to the filler neck. A third vent line has an external funnel shaped external vent on some models and a return line to the engine on others. I am sure there are different variations of the vent system on earlier W113 cars.


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Another interesting feature on this injected sedan tank, is that although the filler neck is in a different position on this sedan tank than a W113, there is a flat area where the W113 filler neck would be. From this I presume that the factory uses the same parts to make  a sedan tank as a W113 tank. Where  the filler neck is installed makes it a sedan or W113 gas tank!

Joe Alexander
Blacklick, Ohio

[Benz Dr.]

Note the term '' flower pot '' was coined by the Benz Dr. and he will permit this group to use it for now ~)

 Now that you can all see what it looks like let me tell you how it works. Right where the return line ends there's a small hole right at the bottom where fuel enters the flower pot when the tank gets low. The high volume of fuel coming out of the return line causes a vortex to form inside of the flower pot and a low pressure area right at the end of the pipe. This low pressure area is right at the hole in the flower pot and actually pulls fuel in from the main tank area.
 The flower pot acts like a dampener so that you don't suck in a lot of air going around corners. Since all the returned full is dumped into the flower pot it never goes dry - there's far more in there than the engine needs.
 This is why it's so important to have an unrestricted return line. Not only does the high volume of fuel prevent vapour lock inside the injection pump but it also acts like a heat exchanger by lowering the internal temps of the IP.
The flower pot is both simple and very effective - just short  of brillant.

Daniel G Caron

[Sphe]

My father and I have decided to undertank sealing our own tank... Now, it has been recommended in this post to close off the fuel lines, as they can get clogged with the sealer... but shouldnt they get sealed as well because they could be rusted on the inside too?

[rwmastel]

Sphe,

You'll be sealing the "outside" of these internal lines.  Does it matter which side, in or out, you seal?  What are the thoughts of others in the Group?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[hauser]

Joe I have enjoyed this topic. It's given us a look at something we never see but now have a better understanding of how it works. Thanks!

1969 280sl 5 spd        1999 ML320          Gainesville,Fl.

[Benz Dr.]

I found that the small screen going into the electric fuel pump can be a hidden source of problems. Since the fuel hose going to the pump is pretty much gravity feed any restrictions will really impeed flow.
Once the fuel level goes below the top of the flower pot there's less weight on the line and the lower the fuel level the worse the car runs if something is plugged. A quick test is to take the hose off at the pump and let some fuel drain out. If everything is working properly the whole tank will drain out. If the small hole in the flower pot is plugged then the flow will stop even though there's still fuel in the tank. ( fuel level must be below top of flower pot for this test ) Removal of sending unit is a must so you can see inside of tank.  A small wire can be bent and then used to poke into the hole if it's plugged ( this is actually quite rare to plug up but it does happen )

NOTE:
 NEVER, EVER, use a trouble light to look inside of a gas tank. If the bulb is loose and you get a spark.....
  Use a well sealed flash light
 ( torch ) only and replace the sending unit as soon as possible after looking inside of tank.

Be smart and safe not burned up or worse.....

Daniel G Caron

[Benz Dr.]

Just a note:
The tanks without a flower pot are for non fuel injected engines. Cars with fuel injection MUST have a tank with this type of fuel dampener or running problems below 1/4 fuel level are very possible. Injection pumps need a constant supply of fuel without any air or vapour bubbles to work properly.
One picture shows the area beside the return line where the hole for fuel entry is located.

Daniel G Caron

[mdsalemi]

NB for those that are interested, the good doctor's (Dan Caron's) observations are fresh, and direct from his work on MY CAR!  This is definitely from "my hands are dirty and covered with gasoline" experience within the past few days!



Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Sphe]

Yes I know I am sealing the outside of the internal lines, but my point is that the inside of those internal lines could be rusty too... correct? or should i prevent any sealer whatsoever from getting inside the lines?

[ja17]

Thanks Hauser,
I am glad these topics are helpful. I always learn something myself from you guys!

Joe Alexander
Blacklick, Ohio

[ja17]

Hello Sphe,
Just use a good brush and scrub the lines inside when finished. Blow with compressed air and make sure all lines are open with a wire. The fuel screen and fuel filter can handle what little rust makes it after the cleaning.

Plan on unhooking the main front to rear fuel lines and blow compressed air through them before putting everything back together.

Joe Alexander
Blacklick, Ohio

[Sphe]

So I DO let the sealer get in the lines? is that the verdict?

Eugene
1965 230SL, Max Speed: Zero

[ja17]

Hello,
I would avoid this if possible, the sealer would restrict the internal diameter by decreasing or closing off the lines. However, I am afraid there may be nothing you can do to prevent sealer from entering the "flower pot" and entering in the fuel return line. Just plan on clearing the holes afterward and avoid as much fuel line sealing as possible.

Joe Alexander
Blacklick, Ohio

[merrill]

Joe,
I wanted to remove the fuel drain plug in my 66 sl to clean the scren and to check for debris in the flower pot.
I tried to use a 7/8 bolt and a vise grip to remove the plug, but that sucker is either stuck or I need a  better tool.  (the vise grip would slide)

should the drain plug require considerable force to remove?
I am sure that the plug has not been removed ever.

any suggestions?
thanks in advance
matt

[Benz Dr.]

I guess I have to give up another one of my secrets.

  The drain plug can be removed using a spark plug socket with a hex head on the drive end. This socket should have a rubber insert used to hold a spark plug which you need to remove.
Push the hex end of the socket into the recess of the drain plug. Fit a long extension through the socket into the square drive from the bottom and remove like a regular drain plug.
CAUTION!!!!!!!! NEVER use a trouble light near gasoline. A loose bulb could start a fire with you covered in gas.
It happens far more often than you might think. Don't be stupid.......

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[A Dalton]

<<spark plug socket with a hex head>>
 
    That's a Keeper .............1/2 drive one would be best, but even 3/8 will do.