saddle tank syndrome!

cellutron

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I wonder, most of the time, about some form of a solution to my problems! (technically speaking, it is the problems associated with my car!). I spend a lot of time reading these and similar forums, but I never seem to get to the bottom of it! A lot of good hearted people write with passion, about interesting subjects on their Mercedes, but the harder I try to see the solution, the less I find helpfull the passionate articles! I must admit, this is not universal; on some small simple issues, there seems to be clear cut solutions.
I give an example, and this is a subject that through out years, nobody has come up with a remedy that is economical, ultimate and explicable! And that is the 'half-tank-run-out-of-gas' syndrome, on, practically, all CDi mercs, though most peculiar to 220CDi engines!
I can understand it is a complicated design- perhaps, stemming from the historical- German -extortion- related practices, to squeeze the last dollar out of your back pocket and force you to prostitution to fund the repairs! The Venturi principle, the saddle tank, the mechanical high and low pressure pumps, the flimsy plumbing set up, the filter monopoly and the many other things that I am sure to find, all lead me to think, there can't be a cheap solution, or any type of solution, but to resort to random replacing of horrendously priced parts and hope that resolves the issue, at a cost that exceeds the market value of your 10 year old vehicle- i.e., what is really done by the spotty teenage trainee mechanic at the main Mercedes dealer, who, very possibly, breaks a few other things or over torques some other bolts and nuts and spits on your enging too!
I am sure if I were to start with the floats and pipes, high and low presure pumps, line sensors, and all the pipes on top, I might eventually end up being able to extract the last drop of diesel from the other half of the tank before the red light can warn me of fuel shortage rather than stopping dead at a junction or in the third lane on motorway, when half- full! Or then again, there is no certainty that any of these actions would get the incumbent saddle tank to do its job like other fuel tanks.

So, I am happy to be a member of this forum and read threads with delight! Only wish that Mercedes were as good cars as they used to be in the golden olden years of the empire!
 

cleverdicky

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Sorry i know little about diesels or your model, but are you trying to say you have 2 separate tanks joined by a pump arrangement.

Or a single tank divided in half, with no means of the fuel getting from one side to the other ?
 

SteveX

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Welcome.

I'm affraid I am in the same boat, diesels aren't my thing I know little about them.

I do know however that it can be quite problematic for a diesel if it runs out of fuel and gets air in the system. Did your problems occur after an accidental running out of fuel or just randomly?

I am affraid that is all I can add :(
 
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cellutron

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saddleTankSyndrome

hi both cleverdicky and stevex

No, it appears that the fuel tank on a 220cdi w203/cl203 ( and may be others) belonging to years past (mine is a 2003, but perhaps newer models as well) sits on the drive shaft, hence the term 'saddle'! it appears that the two halves (that share a small area right in the middle top where it sits on the shaft - mainly occupied by two pipes linking the two float/sender units and the electrical connection between these two) do not communicate to share and balance the fuel level; so the left half always remains full (whether you run out of fuel or not!), but the right side that is attached to the filler pipe and feeds the engine, gets used up. So when the fuel guage on the dash shows half tank (this is the right half that never gets used!) you can in effect run out of fuel (because this is the only part of the fuel that has access to the engine!).
There is a long write up in the forum here about this problem that I have followed and tried to understand. It is claimed that the problem is associated with the low presure half of the pump system that fails to return the excess fuel to the tank and hence breaches the 'venturi' principle! However, somehow, it doesn't make sense though: if assume that is the case, then what happens to the excess fuel that is not used up and has failed to return to the tank?
The tank is not accessible from under as it is covered by the full tray underneath. As such, I am not sure what else can be found under next to the tank in terms of connection/ filter/ an electric pump or else? All the indications are that only the mechanical high and low pressure pumps at the front of the engine are the ones doing the job of feeding the engine and returning the excess fuel.
I know a good manual/diagram can help discover the intricate design features or even answer many questions, and I hope someone has access to that that can publish on this site for prosperity! Many thanks!
 

hotrodder

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Fuel pump sends fuel to the engine and the excess is returned to the tank. This return line is what's (supposed to) provide the pressure differential for the venturi effect to syphon fuel from the 'passive' side of the tank. Kinda along the same lines as a carburettor where differential air pressure is used to pull fuel from the float bowl

Not specific to a w203 and the pic is actually for a variation where the venturi effect is provided directly by the pump (via a small loop circuit) rather than the fuel return line... http://patentimages.storage.googleapis.com/US6907899B2/US06907899-20050621-D00000.png Fuel passing through item 56 is forced though a venturi tube/orifice, physics happens and the fuel at item 82 'sees' less pressure and moves to the other side of the tank to get equilibrium

If the fuel pressure is down for whatever reason (blocked filters/dodgy fuel pressure regulator/tired pump etc) then it won't work properly. A problem with the pipework could also mess things up- blockage at the passive side of the tank due to crud, pipe damaged in some way or adrift etc. Sorry, dunno which, if any, is the most likely/are known to effect 203s
 

cleverdicky

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Well reading that explanation, if i understand it then rather than say 'venturi effect' which I agree is damned confusing, it is relying on the vacuum created when the fuel is drawn up the feed tube.
Or basically a siphon.
What an arse of a system !

So besides all that could block or go wrong, any air leak in that fitting or hose would also prevent it working.
So that might be something to check around the seals that its letting air in.
To be honest, pita, but I would have another pump in the other side. might be possible to gain access through the top (under the back seat)
 

GEORGEROV

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Same system used on Subaru Foresters and Outbacks , never had a problem with the operation but rust affects steel fuel lines and filler connections on older models . Have to dismantle rear suspension and drivetrain to drop tank to get access. Nightmare ! .
 
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cellutron

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Many thanks to all! The detailed explanations are all good, the links to diagrams are even better! It is definitely much clearer! All I need now is a few sunny days to vent my anger at this calamity of a design just for a bit of vacuum!
 

Alex Crow

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Check fuel return flow is OK, then suspect a broken or disconnected pipe in the tank, possibly after a fuel level sensor change.
 

brandwooddixon

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The W210 has a saddle tank as well, but I never experienced this problem with either of my petrol powered vehicles and I would regularly run them down to near minimum (well past when the light came on).

I think that the closest that I managed was to within 2.5 litres (well 77.5 litres to fill on an 80 litre tank).

I suspect that the transfer pipe has become blocked between the two sides of your tank. Perhaps the problem is exacerbated by the oily nature of diesel.

Would a high dose of fuel additive/cleaner help?
Otherwise I suspect that physical replacement of the pipe is required.
 
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cellutron

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I think saddle tanks are quite common as it is a means of utilising the sapce around the prop shaft. What causes this long standing problem on diesel engines, is the extensive plumbing and the mechanical pumps for both drawing from the tank and returning excess to the tank. Even this is not the whole story! The balancing fuel levels in the two halfs is not via a sensible reliable connections; no even the reliable electric pump to do this job. But it is the same culprit: The Venturi Principle! Just imagine the amount of mechanical energy is needed to creat enough vacuum to suck a little fuel through the 2 meters of pipes there and 2 meters back! Assuming airtight connections all round, and a decent lively pressure pumps!
But you know, this discussion has thrown light on a lot of issues that are normally not spoken about! I am much obliged and feel much wiser, yet, I fill the tank before it gets to the mid range, just in case!
 

Alex Crow

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This is not a common problem on MB cars, and most on the street have saddle tanks.
Yes, I know there are cases like yours, but in most cases the system works without fault for the life of the car.

Did you have a look to see if the modules on each side have ever been removed?
 
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cellutron

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hi alex
it is so common that all local non-merc garages quote £600+ as soon as they hear about half tank syndrome!
I have looked at both sender units and contrary to popular belief, they seem to function ok. I can't see the bubble in the pipelines and the engine runs beautifully when filler side of the tank has fuel in it. It just doesn't balance the fuel levels, that's all!
Also, 2 days ago, filled the tank with unleaded!!! Not realising, I drove off and just a short distance away from the petrol station, it just died! I have drained the tank and taken most top pipes out, including the low and high pressure pumps. The low pressure pump seems and feels good and squirts out fuel. But the high pressure, when turning by hand, feels quite stiff! I plan to take it to a diesel guy to test it/ feel in and give his verdict today. I hope to hear that it is faulty so at least I can celebrate the finding of the true criminal for the saddle tank syndrome!
Many thanks
p.s.; I do believe mercedes are good cars despite annoying things such as this!
 

Alex Crow

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I have looked at both sender units and contrary to popular belief, they seem to function ok. I can't see the bubble in the pipelines and the engine runs beautifully when filler side of the tank has fuel in it.

Did you check the return flow?
Did you check the cross pipes were all connected correctly on both sides, and not kinked?
Did you check the venturi nozzle was in place correctly?

More very specific details and less words would help us help you - answer the questions please.

If the petrol was only in for a mile or so, no damage will have been done, but maybe you have done this before without realising (many do), and already damaged the HP pump...
 

Steve@Avantgarde

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I would say the likelyhood is the high pressure pump is at fault. I had a C220cdi in on tuesday with this exact same fault. If the Low pressure pump is working (driving the vacuum to pull the fuel across) and pulling the fuel to the engine when there is supply then all the other pipework must effectively be ok. It only leaves the high pressure pump as the common denominator which, if the return valve sticks blocks the rest of the system from working.
 
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cellutron

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dear alex
sorry if i missed out the preliminary details! all the common sense ources have been covered.
no, i havent checked the return flow, but intend to do as soon as i start the car after the misfueling.
inside the tank, the pipes are ok. as they are hard plastric, there is no chance of kinks, or disconnection as the only way to disconnect is to cut and replace! they are very tight fit.
venturi nozzles? where?
my local diesel specialist suspects the o rings on the injector leak pipe and the fuel line solenoid o ring. the pressure pump is said to be the least likely cause and it seems to be ok.
manmy thanks. will update in due course.
 

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It sounds like the fuel tank breather is creased/blocked. It only vents from one side so this may well cause the symptoms you describe.

Loosening the filler cap may help temporarily, but I would be inclined t find the pipe itself.

btw, the idea of the saddle tank set-up is to allow folding down of rear seats or ski hatch.
 
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cellutron

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this is a good point! i ve been meaning to check the breather for sometime now, but, i have no idea where it might be! strangely enough, only a few days ago i noticed a pipe hanging from the rear wheel arch area, where the filler pipe is, but behind the wheel! I am sure it wasn't there before. so, it is in my plan of actions to do when i get the engine running after the mishap misfuel a few days ago. I think a new thread is in order for that so it goes like this....,
 


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