Mercedes W114 250 Coupe - help!

[Charles Morgan]

If the rail has sufficient volume (twice the original's? - easily measured) then feed one end and if the mounting is all good and the regulator not to heavy - mount the regulator on the other end?

The replacement rail has a volume about 2.5 times that of the original so that would work well.

 

[Charles Morgan]

Is the swirl pot definitely in the supply line (and not the return)?

It's hard to tell from the EPC diagram, although it looks to be after the fuel pump, and I have no photos, but the EPC description is damper cage in English, Daempferbehaelt in German, which damper jar comes back in Google, my inclination is to translate that as a sort of shock absorbing/flow regulating pot. Whatever that is...

 

[Bellow]

Or pressure accumulator/pulsation damper?

 

[Charles Morgan]

I'm reminded of the Yes Minister meta-dioxin plant episode...

Possible. With a modern fuel pump I doubt anything in between is necessary.

 

[Bellow]

Pressure accumulator good!
Maintains line pressure on switch off for prompter restart and prevents vapour lock in summer, and smooths out the pulsations from the pump and from the injectors dumping rail pressure three injectors at a time.

More rail mounting thoughts later.

 

[Charles Morgan]

With a modern fuel pump I doubt anything in between is necessary.

Other than a fuel pressure accumulator of course. Not having one would clearly be a brave decision Minister...

 

[Bellow]

Accumulator and filter should be enough. Could the 'swirl pot' be a charcoal cannister? I think by the time of the W124 MB were fitting them but when they began the practice I have no idea.

 

[grober]

Most D jetronic sytems simply depict an inline fuel pump followed by a fuel filter which would provide a certain damping effect anyway. the pumps are the internally submerged type with fuel cooling the innards rather than the fully submerged type. The later K and KE JETRONIC had fuel accumulators/dampers to maintain system pressure for a time after switch off but I reckon the importance of a steady pressure was greater in the CIS [ continuous injection system] where metering was perhaps more dependant on a nice steady pulse free supply pressure than the individually pulsed D jetronic or later MOTRONIC sytem- witnessed by the disappearance of any damper/accumulator in the later M111 or M104 engined cars ???

edit:- further reading reveals the presence of a pulsation damper in the fuel return line in some Motronic sytems --- post the fuel rail pressure regulator on the return line to the tank but and perhaps this is the important bit not at the supply end of the SYSTEM i.e. [ pump filter rail }

 

[Charles Morgan]

Here's the EPC diagram for my chassis number

 

[grober]

Take a look at the second EPC picture Charles 1--2 which depicts a much simpler set up [ e.g. the fuel filter is depicted unlike picture one where it doesn't exist and what I think is also a picture of the mechanical fuel pump for a carburettor setup ??

 

[Charles Morgan]

Diagram 1 is my car set up - filter cover is correct, shape of fuel pump etc - the chassis number also falls into the range specified. The fuel filter is shown in 07 Injection in the Engine Maj assy section.

In a sense this is merely archaeology - the correct question is what is needed on the new system.

 

[grober]

All I can offer is that with the D jetronic the technology was in its infancy and perhaps there were certain belt and braces aspects included which were found not necessary later. [ that double ended fuel rail feed for example] That 3 outlet fuel pump also- in some setups there appeared to be some form of bypass back to the tank- I can only assume this was via some sort of internal over pressure valve which allowed access fuel pressure back to the tank but later was found unnecessary- hence the inline pumps with just an inlet and outlet ?
Anyway as you say its historical and best to concentrate on the present setup.

 

[Bellow]

The tendency is to simplify as time goes on so some aspects of D-Jet probably are overdone.
Crib from current practice as the MS as far I know aims to emulate modern day systems.

 

[grober]

Going with the principle imitation is the sincerest form of flattery then copying/modifying a MegaSquirt set up on a similar straight six engine with around the same horse power would seem sensible. No sense in re-inventing the wheel especially since its often slight tweaks to the set-up that yield a smooth idle power transition for example. The citroen setup may be useful in this context??

 

[grober]

Charles it would be useful to publish the model number and specification of the proposed new injector you pictured?

 

[Bellow]

I think, with extensions, it should be possible for the three bolts in the head that lock the injector holders in place also to hold the fuel rail in place - via a hollow tube tapped into the rail billet (but not into the fuel line). That would solve locking it into place.

Can you elaborate Charles on the above?

The idea of retaining the retaining bracket for the injectors I like. I'm just not seeing a method of attaching the rail to the stud.
NB. There is much greater wall thickness in the rail than I originally credited it with. I may have prematurely and needlessly been dismissive of tapping into it.

 

[grober]

A reminder??? http://www.mbclub.co.uk/forums/engine/201721-sizing-fuel-injectors-etc.html

 

[Charles Morgan]

They are Bosch 0280150778 injectors - if the table is correct they are off the BMW V8s of 3-4.5litre capacity, high impedance and 3 bar pressure, with a cc per min 0f 191 (vs about 160 theoretical). Researching my Citroen SM injectors these are also oversized vs theory by about 50% yet no issue with idling and no real issue with fuelling and power. I'd rather oversize a little (yet to find many injectors on the table that exactly match the theory - most are oversized) if idling is no issue. By taking them from an engine I know, which also has EPC details, I was hoping to piggy back fuel pump spec and setup. I would have used the BMW E39 straight six injector setup (an engine I know) but the listed pressure was 3.5 bar.

 

[Charles Morgan]

Can you elaborate Charles on the above?

The idea of retaining the retaining bracket for the injectors I like. I'm just not seeing a method of attaching the rail to the stud.
NB. There is much greater wall thickness in the rail than I originally credited it with. I may have prematurely and needlessly been dismissive of tapping into it.

At present there is a thread on each of the three studs that is used by a retaining nut and washer to hold the injector bracket in place. I envisaged replacing the nut with a hollow tube with a thread of the same size as the nut at one end, and then a stud coming out of the solid part of the fuel rail that seats in the hollow tube. Securing it by thread would not work, but perhaps some sort of clevis pin through the tube and stud to hold it in place - allowing a small degree of movement.

Thoughts welcome.

 

[grober]

I assume that things are complicated by the fact the original system D jet fired two blocks of 3 injectors at the same time. I assume that the new set up will be firing individual injectors and this might explain the discrepancy in cc/min?