saddle tank syndrome!

[cellutron]

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.

Steve, there are 2 ports on the lefthand side of high pressure pump; one receives the T-junctioned- pipe= feed from low pressure pump terminated with a stop valve, and the other port is the return path that connects to the solenoid end of the fuel line. I can't remember which goes where when I took out the pumps and pipes for inspection. There seems to be no arrows or any other indications to show in/ out flows. Can you help please? Thanks

 

[cellutron]

Just to update on this subject (threads tend to end up in the black hole after a while with no conclusions!); the latest advise is to replace the injector leak pipe and the line solenoid o ring. As the leak pipe is some 100s of £, I find the cheapest option is just to fill the tank when the guage shows half empty (or half full on the optimistic note!).

Not wanting to upset the merc enthusiasts, including myself, but I wonder why something that is not affecting performance and general operation of the design, should create such a nuisance?

 

[Alex Crow]

Not wanting to upset the merc enthusiasts, including myself, but I wonder why something that is not affecting performance and general operation of the design, should create such a nuisance?

As has been said, this is not a common problem - just your problem.

Is the leak off pipe leaking? If not, changing it will not help.
Seals on the pressure reg valve will not give you your problem either.

 

[Steve@Avantgarde]

Im glad this thread popped back up and also apologies for missing your other question.

I had a C220CDI sport coupe in on monday which was running out at half tank. After checking all the usual suspects like HPP, alignment of senders I was a bit stumped. I found that as the engine was running the left hand side of the tank was filling up and no transfer taking place to the left hand side.

To cut a very long head scratching story short I had a couple of spare sender units from a breaking car here so decided to swap them to see if it would make a difference and hey presto, fuel started to swirl and transfer properly to the right side of the tank.

I havent had a chance to play with the old senders yet to see if there is some kind of valve or chamber which is blocked, damaged or failed but when I do I will post it here.

Maybe this is the same fault with your car?

 

[cellutron]

Hi

I think the most constructive resolution so far has come from Steve! Many thanks for that! You know, after following all the advice on this subject, I doubtfully came to the same conclusion that there must be some weird and wonderfull thing going on with the senders, though never had the courage to change them as it seemed that the linking pipes had to be cut to remove them! I did however, manually moved the float and noted the fuel gauge follow sympathetically- and furthermore, thought that the thick plastic construct of the senders can't just breakdown! this made me doubt that the fault might rest with the senders!
Now, I shall wait for your findings on the culprit senders and then am going to attempt replacing them, perhaps with known good second hand ones.

As regards the dearest Alex's optimistic view, I beg to differ! Maybe the problem is not so discerning on the not so aged ones, but it is certainly prevalent amongst the rough old ones we cherish!

 

[cellutron]

Sorry Alex, I forgot to answer the questions! No, the leak pipe is not leaking fuel.. That latest advice came from a local diesel specialist who suggested that the air leak is the cause though fuel may not leak at all! Though it sounds odd, but, as it came from a specialist, the only logic for that to happen, I thought, must be the action of suction that might be going on in the closed loop of return to tank for the venturi effect to take place! But then again, if there is an air leak, there is no vacuum(venturi) and hence there should be fuel gushing out! But but but ...I don't know, I am all confused now!

 

[Alex Crow]

Enter stage left, a C220 in for glow plugs, that we had idling to warm the engine.
After 20 mins it suddenly cut out.
Lots of air in the fuel system, with it pulling only a tiny amount of fuel from the tank - too much air to start.
A look in the O/S of the fuel tank shows it empty, gauge showing well over 1/4 full.
Add 2x gallons and car runs again for 10 mins or so, and cuts out again.
A check in the tank at the return flow to the O/S of the tank shows nothing at idle, with good flow above 1200 rpm or so.
Fuel filter is new, no leaks anywhere in the system.
With checks made at idle against another car, primary pump flow is equal on both, but high pressure pump low pressure overflow to end of rail is about half on our problem car.
High pressure pump replaced with a new one and return flow to O/S of tank suddenly v.good, with all fuel scavenging across - happy days.

This is a long story short, as we also elected to check both fuel delivery modules in the tank, and all pipework runs/routing etc before fitting that expensive new pump.
Thanks to Steve for the pointers -

 

[alexanderfoti]

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.

No saddle tank on the W210:

The one with the whole is for the ski schute option.

 

[tode]

Just a little aside: diesel engines seem to require a higher degree of cleanliness than petrol engines. (Maybe due to the higher viscosity).

I've found that dirt and/or condensation are often the cause of running problems. The pumps, lines and filters do not take kindly to the presence of water in the fuel, which is why it is a good idea to keep the fuel tank as full as possible and not to leave a diesel car standing with half-or-less fuel in the tank.

Hope you get your problem(s) sorted.

 

[cellutron]

actually, diesels are like prostitutes- they run on anything! the only thing seems to be needed, is a bit more pressure to vaporise to combust- so need tighter tolerances in their design!
so anything from the float arrangement in the tank, to the rest of the plumbing, and in this most recent illicited case, the pressure pump and all the auxilliaries, where essentially build up of vacuum and loss of pressure is caused, can lead to starting/ running issues. one has to try deliberately hard to introduce dirt into the system as, when running, it is almost a closed loop circuit.

 

[tode]

actually, diesels are like prostitutes- they run on anything! the only thing seems to be needed, is a bit more pressure to vaporise to combust- so need tighter tolerances in their design!
so anything from the float arrangement in the tank, to the rest of the plumbing, and in this most recent illicited case, the pressure pump and all the auxilliaries, where essentially build up of vacuum and loss of pressure is caused, can lead to starting/ running issues. one has to try deliberately hard to introduce dirt into the system as, when running, it is almost a closed loop circuit.

In theory that is more-or-less correct.

But if you use dirty fuel, it just won't run for very long.

Diesel pumps do not appreciate water in the fuel.

 

[cellutron]

agreed!

 

[mercedes13156]

Hi there,

My W203 C220CDi had the exact same problem. It was "running out of fuel" at 1/4 tank. I handed it to my local garage, (who I trust implicitly), and they thought it was a lift pump, or the self levelling kit for the "saddle tank". They opened the tank and found that one side was full up to the saddle and the other side was dry. They also tested all of the tank components and found them working perfectly.

Long story short... The high pressure fuel pump was knackered and the explanation was that it wasn't returning fuel from the rail / bleed off to the tank.

I replaced the HP pump and I've never had a moment's problem since. (Well not with that particular problem). My wife has run it down to zero miles to go a few times and never an issue.

I note that several contributors have alluded to this remedy. It worked for me.

 

[cellutron]

failure of high pressure pump is much easier to spot than the faulty senders in the tank! the latter can look deceptively functional and only a replacement with new proves the fault!
there 's a good feedback on this sometime ago- but the follow up to explain 'the why' and 'yhe how' they become faulty hasn't come yet! there could be no damage to the plastic, no broken resistor track, no damage connecting hoses or wires, etc. somehow they manage to stop feeding the other half of the tank! a costly replacement with new though solves the problem!

 

[Alex Crow]

failure of high pressure pump is much easier to spot than the faulty senders in the tank! the latter can look deceptively functional and only a replacement with new proves the fault!
there 's a good feedback on this sometime ago- but the follow up to explain 'the why' and 'yhe how' they become faulty hasn't come yet! there could be no damage to the plastic, no broken resistor track, no damage connecting hoses or wires, etc. somehow they manage to stop feeding the other half of the tank! a costly replacement with new though solves the problem!

I do not really subscribe to that- any fault inside the tank with the scavenging can be visually checked easily enough (once you have access).

Why not follow all the advice on the thread, and get yours fixed?
Or book it into one of the indies that knows about your issue?