MSefi.com read only forum (Jan.15,2018)

My W116 280SE (M110) being converted to MS Efi

I am using back the original Bosch D-jet injectors and now I am facing an issue with the fuel pressure regulator

The D-jet original Fixed Pressure Regulator(FPR) is fixed pressure(regardless of manifold vacuum).

MS recommends "Vacuum Referenced Fuel pressure regulator"(VRFPR) which will increase the fuel line pressure on hard acceleration...

I read that if I were to FPR, the injectors will have to stay open longer, generating a lot of heat and possibly fouling the injectors.....some claimed that tuning will be difficult with FPR, is this true??

I like the idea of VRFPR but I am worried that on hard acceleration, the fuel pressure may be TOO high for the D-jet fuel delivery system (damaging the D-jet injectors and possibly bursting the soft rubber hoses).

Using VRFPR, I am thinking of setting the pressure at max 3 bars(I read that Bosch claimed that the D-jet can hold 3 bars) but at idling, the fuel pressure may be too low, injectors have to stay open for longer period(thus fouling them) and insufficient atomizing.

Let me know your thoughts.

Thanks in advance.

NPK

Hi
The objective of the manifold-pressure referenced fuel pressure regulator is to keep the differential pressure over the injector constant.
This will certainly help you tune a VE table.
At low pressure="high vacuum"= low load you will need short opening times of the injectors. If you use a constant pressure regulator at say an constant pressure of 0,42 MPa ( quite common) the idle of say o,3 MPa manifold pressure abs, the openeing times of the injectors will have to be reduced substantially. This may give you worse idle, since injectors are fairly inconsistent %wise at short openeing times ( < 1,5 ms) It will also give you much increased granularity .

The highest pressure with a manifold referenced FPR is at priming when the manifold pressure equals ambient pressure.

So, no need to be concerned with leaks as long as you make sure that fittings and eventual hoses are good.

A common failure of te early MB systems with Djetronic ( typically the V8 350 SE engines in the 70´s) was cracked hoses between the fuel rail and the injectors

So make sure that you inspect all couplings and hoses, including those aroud the fuel tank/pump.
And please, do rebuild the Fuel Pump 12 V supply, taking advantage of the excellent and fail safe FPR MS circuitry.
If your car has an automatic gearbox you may need to retain the existing FP-Relay which doubles as electric input signal to the tranny.
Just leave i in place , but move the FPR supply wiring over to a new separate relay controlled by MS.

Best regards and a real nice engine you have there!

Heribert

npk wrote:My W116 280SE (M110) being converted to MS Efi

I am using back the original Bosch D-jet injectors and now I am facing an issue with the fuel pressure regulator

The D-jet original Fixed Pressure Regulator(FPR) is fixed pressure(regardless of manifold vacuum).

MS recommends "Vacuum Referenced Fuel pressure regulator"(VRFPR) which will increase the fuel line pressure on hard acceleration...

Are you sure it says just that? A standard vacuum referenced regulator only reduces fuel pressure from its nominal rating. This is to nullify the 'suck' effect on the injectors under conditions of high vacuum. Of course there are aftermarket 'rising rate' types - but there's no point in theses with an MS, since you can replicate their effect in the settings.

I read that if I were to FPR, the injectors will have to stay open longer, generating a lot of heat and possibly fouling the injectors.....some claimed that tuning will be difficult with FPR, is this true??

I like the idea of VRFPR but I am worried that on hard acceleration, the fuel pressure may be TOO high for the D-jet fuel delivery system (damaging the D-jet injectors and possibly bursting the soft rubber hoses).

Using VRFPR, I am thinking of setting the pressure at max 3 bars(I read that Bosch claimed that the D-jet can hold 3 bars) but at idling, the fuel pressure may be too low, injectors have to stay open for longer period(thus fouling them) and insufficient atomizing.

Let me know your thoughts.

Thanks in advance.

NPK

If you go for a standard type and it regulates at the same pressure as your original there should be no leakage etc problems.