What is it for?

Please post the model and year anytime you ask a question (or if you have one car, put it into a signature line that appears automatically).

Assuming this is a w126 or w123 turbo, the shape of the cigar line is designed to dampen pulses in the return line. In theory it is one factor necessary to ensure a smooth idle.

There is no serious detriment to replacing it with standard fuel hose if the need arises, but given that it's about $11-13 from online suppliers I choose to use the correct part myself.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

some say it dampens pulsations on the metal fuel return line it is attached to.

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1981 NA 300D 310k miles

I seem to remember reading somewhere that it was designed to dampen the return pulses for the purpose of preventing fatigue in the metal return line...

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

Hmm, i kinda thought it was the case, Thanks

On a 123 type diesel fuel system in good shape. The relief valve is always open when the engine is running.

The cigar hose then mitigates very sharp pulses present and generated in the injection pump. Or reduces them at least in amplitude signifigantly. Probably even them transiting further backwards to some extent in the fuel system as well.

These hydraulic pulses are so strong much has been made of them being audiable sometimes as well.

Running an engine that cannot sustain enough fuel pressure to keep the relief valve open is something that should be checked out I believe. Either the filters are too restrictive or the lift pump has grown weak with age is the probable causative.

There are many cases of these engines developing weak relief valves with age. Few of the relief valves being obstructed if any. The way the injection pump is calibrated it is perhaps wrong to run without a cigar hose or too low of a supply fuel pressure. They seem instead to run better overall when maintained as designed anyways.

Much evidence at least to me is that this is a cheap and easy system to maintain properly. Not doing so can destroy or seriously damage an engine for all practical purposes with time. I personally over time have developed this as a belief. No absolute proof though. Yet still I am pretty certain.

Some cars will also display some undesirable effects when the cigar hose is replaced with a hard line is proven though as well at the same time. My current guess is that if the relief valve is not seeing enough fuel pressure in the system to remain open when the engine is running at any speed. The system is defective. Simply because the cigar hose is being denied its design function. Plus there are probably lots of examples doing precisely this out there.

Also if the tendency of the relief valves spring to weaken with age had not occurred. There would have been many more failed engines in our time than there have been so far.

Too many people still feel that a diesel engine of this type. The 616 and 617 require almost no maintenance. The reality is they do require some and it is neither expensive or difficult to do. If the cigar hose has gone hard with age is one of the simple things that need checked as well.

Last but not least. The 616 seems to me to be far more probable to fail with an out of design operating fuel system. The car will seem to run okay but will ultimatly wear out the number one rod bearing with time.

It takes longer to occur with the 617. This primarily because the lift or fuel pump is a higher pressure design. I used to think it primarily was because of the greater amount of power strokes as well.

Anyways the failure mode on the 617 seems to be either the number one or number two rod bearing with fuel system neglect. I know some of these engines go 500k is a reality just from the amount of time a high percentage of either odometer rollback or defect is indicated. Plus some of the general condition of some examples. They will never see extremely high miles in my opinion with a basically totally neglected fuel supply system though. So to a simple person like myself the cigar hose is an important component of the system.

I got to the stage quite some time ago of thinking ownership of a zero to thirty pound fluid dampened ten dollar pressure gauge should be owned by many. Otherwise one does not know exactly what his or her fuel systems condition really is. These are old cars remember and can deteriorate with age alone.

As I was typing this a quick test evolved in my simple mind. On a system I suspect you should feel the pulses in the cigar hose simply by grasping it if the relief valve is open. Although at the same time I wondered if the injectors return pulses where present as well to default the test. The injector return is miniscle in volume though by comparison.

People should express their opinions in this area as well. There is always something to learn.

Good idea, thanks

Man, that's fascinating how they polished every detail. Thank you.

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General George - 1967 Land Rover 2a SWB 1983 OM617 Turbo

My guess is the design intent was to have constant fuel pressure available in the injection pump. Rather than an expensive input pressure regulator they decided on a constant overflow controlled by a much cheaper relief valve.

Plus they obtained some much needed cooling effect as well with the constant fuel overflow. The produced heat by the injection pump was then dispersed in the fuel tank. This also solved the changing viscosity of the fuel with temperature issue or at least minumised it.

My other guess is they ran into problems from a calibration perspective from the strong hydraulic pulses produced by the injection pump elements. These would also vary in amplitude by the reverse fuel gravity pressure loading the return from different levels of fuel in the tank.

So the best way to make dealing with the excess pulses a constant was to incorporate the cigar hose in the output of the injection pump circuit. This would at least dampen them substantially.

I suspect this injection pump was engineered in the 1930s as Mercedes introduced the diesel engine to their production then. The injection pump was improved with time in some ways. At the same time it remains somewhat 1930s technology.

Actually to me a pretty remarkable and workable design with good lifespan for the times in my opinion. There where no cnc type machines back then either. So these pumps where almost a work of art to me. Or at least I was always impressed.

The little pistons and their cylinder liners probably where individually hand lapped to get the needed precision fitting required back then. Even today this still may take place in an automated fashion. Although my knowledge is limited as nobody makes an in line piston injection pump anymore unless perhaps for the 616 being produced in India perhaps.

I never have seen an explanation why export versions of Mercedes to north America have a different injection pumps than the true european cars.. Part of my suspecting failures might be fuel supply related initially. Was derived from their lack of the same issues. Many European owners claiming they do not have the same failure issue with there otherwise identical engines or have ever even heard of it. Their diesel fuel there is much better than ours is the only other variable I can think of. If that were the case though the injection timing on our fuel would be earlier than their engines or should be.

i'm thinking that if it was meant to absorb injector return impulses it'd be located between the injectors and fuel filter.
True the pressure there would be greatest but the volume not so much. It seems to me that it's intended to countermeasure the fluctuation of pressure in the IP created by the IP's working pattern for a lack of better word.
It does seem important though.

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General George - 1967 Land Rover 2a SWB 1983 OM617 Turbo

The hydraulic pulses generated inside the injection pump by the elements are so powerful. Some injection pumps had strong steel shields installed for them to impact against. Rather than slowly erode the softer aluminium pump casings themselves. .

Not the 616 and 617s though. My first guess was that the designers of those other pumps did not come up with the solution that was used on our models of injection pumps.

Although I know really very little about injection pumps must also be taken into consideration.

What I also suspect though is when almost total havoc is present with no moderation of the intense pulses. The individual elements cannot manage to reload the same as when they did with a properly working system.

So some cylinders in the injection pump seem to consistantly get more or less fuel than they were designed or calibrated to load. The variable that makes this a possible constant is the lift pump is a constant pressure design. Still it sags down a little during the sequential loading of the elements if the relief valve is closed. This occurs because it is only reloaded once every injection pump rotation. No problem with the relief valve working though. As that is the injection pumps fuel pressure regulator a steady constant. Allowing the cigar hose to do its moderation of the pulses as well.

To sum it all up basically low fuel pressure continued over the years and miles is a slow death sentence on these engines. Not changing filters when partially plugged for example can cause this as well.

Plus the engines are less powerful. Have a poorer idle than easily possible etc. Some people have even reported less engine noise at high speed after addressing a faulty system. This may or may not be true though. As we are looking for changes after a repair. Imagination can easily play a part of what we think.

For example you only usually know the car can accelerate better from a seat of the pants feeling. Or may notice you are not so far into the pedal as you where before correcting the system.

I personally like to see 19 pounds operating fuel pressure. It does no harm and may even extend the engines life by also reducing the loading slightly on the first and second engine cylinders. That's if they have been subject to overloading in the past with a too low a fuel pressure for years. You may be somewhat equalizing the wear eventually as well this way. What you will have accomplished is a far better power balance inside the engine is pretty certain.