Main.TrailIndexPage | Fuel System | Fuel Tank | Fuel Venting System

Fuel Venting System

This component is part of the Fuel Tank.

Fuel Expansion Tank - Small (Beluftungsbehalter Klein)


A small Fuel Expansion Tank was fitted to most European, and early North American cars, to safeguard the fuel system by providing atmospheric venting of the fuel tank.

It vents the Fuel tank, and when the tank is completely full, it acts as an overflow reservoir.

Fuel/Vapour from the Fuel tank can come up to the Expansion tank through the two forward lines and can return through one or both of them. The rearmost line vents to underneath the car terminating with the FFD that aids extraction.


The small plastic Expansion tank is located and fixed in the right rear of the boot (Trunk). It has two 5mm tails for connecting to the fuel tank, and one 6mm tail for exterior venting.

The Fuel tank has two 5mm tails protruding forward of the filler neck, these are concealed under the filler neck cover. The cover is not an airtight seal, and is fixed to the boot (trunk) floor and rear panel, by 10 screws.

Two 5mm steel pipes run from the Expansion tank to the Fuel tank, and a 6mm steel pipe runs from the Expansion tank to the underside of the car where it terminates with the Funny Funnel Device near the tail pipes. All three pipes run through the filler neck cover.

All the tails are connected to the steel pipes by short pieces of flexible fuel hose/pipe.


Imperfect sealing of any of the connections can result in fuel fumes being drawn into the trunk (boot) and under certain circumstances the cabin - especially when driving a closed car with an open window.

There is some debate as to the correct size of fuel hose, and which connections should have hose clips fitted.

DIN 73379 Over Braided Fuel Hose 4.5mm I/D - 9.5mm O/D secured by 9mm spring clips gives a leak free connection for all the 5mm tails. NB 4.5mm equates to 11/64” - not 3/16” I/D pipe.

N.B. 3/16” fuel hose without clips will not provide a vapour tight seal on 5mm tails.

The 6mm vent can be connected using 5.5mm I/D similar hose.

Blockages in the system can result in Fuel problems, due to the Fuel tank being unable to vent as the fuel level drops in useage.

paladinJune? 2015

Parent page Fuel.Tank

Old Yahoo content

The following is the content from the old Yahoo documents on the site. It needs to be structured and edited in the correct sections of the entire document. After moving particular content to its correct place in the manual, please delete it here.

Venting system

Expansion Tank (for fuel tank ventilation) is located on the right inside fender of the trunk (1967 US model 280 SL). How should this expansion tank be properly connected to the fuel tank? There are three outlets on it. On the fuel tank there are two outlets on each side of the filler nozzle. Thanking everyone for the advice.

Frank says: Without looking, I'd say that the two smaller ones should be connected to pipes that connect to vents at the filler neck & center of tank, and the rear larger one connects to a pipe that vents to the atmosphere, emerging just to left of the tailpipes.

I seem to get excessive pressure build-up in my fuel tank when the tank gets warm. Is pressure NORMAL? Shouldn't pressure vent through the evaporative control tank out either the crankcase or atmosphere vents? I've tested the valve with light air pressure and vacuum and all three valves open or close as they should. Vent tube to crankcase and atmoshpere are clear as are flow and return lines to and from the fuel tank.

I had the same problem. Turned out to be the gas cap. I got a new one and the problem disappeared. The gas cap is also vented. Mine was plugged somehow. This may not be your problem; see if you can borrow a good cap from someone and try it. A new Mercedes gas cap is expensive.

We went on a 60 mile trip last week in Colorado with 95+ temps stopped by the store before going home and it would not start up again - lots of pressure in the gas tank ("vapor lock")? After sitting for 20 min it was fine. Wouldn't the tank in the trunk handle excess pressure?

Illustration from Haynes... shows how it works and I can't see how it can create pressure in the fuel tank unless there is lots of fuel in the compensation tank. My understanding is that the compensation tank would only have minor amount of fuel in it when the fuel tank is full.

Trunk board in a 1971 280 SL. I ordered the driver side board to hang the fuel vapor tank but I think I received the passenger side board instead (Part 113 690 01 35.) Am I correct that the two boards are different? The board is about 6 inches higher than the side of the trunk. If I push the board against the wall the top wants to bend in along the inside top of the fender, is this correct? A little concerned because the board seems stiff top to bottom. I'm afraid the board will crack if it bends in. Should I just push slowly? Also does the thin foam sheet go between the board and the wall? Finally: there are three rectangular holes in the foam sheet top to bottom near the trailing edge, shown in the diagram in the orange parts book (Table 29 of the 1968 catalog). These holes don't seem to have any purpose. Is my car missing something or are these holes used for another car?

That board is sold new in flat form, and one side has a foam covering. The side with the foam should face the sheetmetal side of the fender as you put it in, you will need to curve the top so it will cover not only the side but the top too. The difference between the two sides is obviously the side the foam is on and the holes punched out for the evaporator tank clips. There are two different kinds of evaporator tanks that I know of, the large one for later models and the smaller one for the early models. Make sure you tell them which one you need.

The board does curve quite sharply following the lines of the rear quarter panel right up to the lip around the trunk opening. Just keep pushing once you have it in place at the bottom edge. The material is quite tough and durable.

These cars have venting in either direction. If the ambient temperature rises the fuel expands and so the pressure has to be relieved and as the car runs air has to get into the tank to replace the fuel or the tank would implode.

If your car is like mine, you can check out the vent lines by removing the tank filler cover located inside the trunk. You will see the 2 small hoses clamped there. The hoses run along the rear of the car and then to venting tank on the left side of the trunk.

It seems to me that pressure build up in the fuel tank could be caused by a faulty pressure relief valve. On my car, there is a plastic line coming out of the top of the compensating tank. This line connects to the venting valves located in a round metal cylinder mounted just in front of the compensating tank. There are 2 lines coming out of this valve cylinder, one goes to the engine for recirculation, the other vents to the ambient air. If you have the Big Book, it is explained in Job 47-2. I would try disconnecting the line at the top of the compensating tank, prior to entry into the valve cylinder, and see if that helps.

This does seem like a difficult phenomenon to explain. On my car, a 1969 280SL with a 4 speed; originally delivered to Pa. (USA Specs) there is a very small (not much larger then my fist) plastic container under the passenger side rear fender. There are two lines coming in from the filler neck to the bottom of the container and a third going from the bottom to vent out underneath. There are no valves that I know of and no connections to the engine. I recently checked again, driving my car and then opening the cap. This time the tank was about 1/4 full and I had no pressure. I attribute my experience to 1) when the tank is full, fuel can collect in the container forming an air block until the level goes down, or 2) I had some kind of blockage which I cleared out by blowing through the lines.

I don't have your configuration. I have two lines coming from the fuel tank to the bottom of the black plastic compensating cannister. I then have one line coming from the bottom of this cannister that routes down through the trunk floor and vents out through the funnel shaped rubber thing by the exhaust. I do not have another "valve cylinder". I do have on top of my cannister what appears to be another connector, but I can shine a small pen light in there and see that it's moulded shut.

I dug into this today. I have the small tank that Rodd and Chuck have. I checked the Big Blue Book and it's in there, job 46 or 47 I believe. It describes both systems (the small, basically non-emission control canister and the larger later US-canister). The book says that the small canister

  1. vents the gas tank (hence the non-venting "ohne beluftung" gas cap - it does not need to vent because the little tank does that) and
  2. when the gas tank if completely full, it acts as an overflow reservoir.

Gas can come in through the two lines and can return through one or both of them. The outermost line vents to underneath the car. What I did not know before looking at the picture in the book was the proper position and place - mine turned out to be installed 90 degrees wrong, i.e. instead of having the three lines on the bottom, they were on the side! Fortunately, the venting line was on top, so it still functioned o.k. Still, I am glad I looked into it and changed it. There was a small amount of gas actually in the canister, which was completely full when I started looking this morning. The canister itself is made from plastic, I am sure it is empty (e.g. no filtering material or valves). It has two small protrusions on the top and on the bottom and these are closed, no opening possibility. I think they may serve for affixing the canister to the inside of the fender. Basically, I think that if you can blow through all three lines unrestricted, the thing works. If all lines are free and they and the canister are installed properly, and you have the correct non-venting gas cap, the only other cause for gas starvation on this type of (non-emission controlled) set-up would have to be elsewhere in the system, e.g. the pump, fuel pressure regulator, filters, lines to the engine etc etc.

The previous owner covered all the rubber tubing lines going to the plastic fuel fume tank in the trunk with undercoat. The undercoat is tacky. If I try remove the coating with a paper towel, the undercoat doesn't clean up but the paper towel sticks to the tubing. What can I use to clean up these rubber lines???

Use a suitable cleaner like goof off or a kerosene soaked rag to wipe off the undercoating. Again be careful with the use of any flammable and do the work in a ventilated area. Try to be quick with the cleanup because the kerosene will degrade the rubber parts, if left on too long. Use a degreaser when finished removing the undercoating mess off the lines.

Another option: new hose. Not very expensive, but only sold by the meter from MB dealer. Lots of extra hose that way.

Pete Lesler: try PB Blaster. It should be available in any autoparts store. It is also the best penetrating oil for breaking free rusty bolts.

On this same note there is a company called Eastwood - you can go to their website at and find a product called ‘under gone’ it is supposed to remove undercoating. The rest of you might like some of the other products that they sell. And no I am not affiliated with them in any way. I have just purchased from them in the past and have been very satisfied.

< Fuel Tank | Main.TrailIndexPage | Fuel Sender Unit >