The pressure in a Djet remains constant.It doesn't rise or fall unless the pump is failing,the filter is blocked or the the return is blocked. At idle it should be 28 PSI,when acelerating it should remain at 28 PSI.
The original setting of 27 PSI is within specs and I would turn it back to that.
As I said,those injectors will leak if the pressure rises to much,as it is now.
Modern fuel differences are not responsible for injector faults*. The Djet injectors actually flow more per second than most modern injectors can and by increasing the fuel pressure to a point outside the design specs you have in fact created a problem and are probably increasing your fuel consumption and causing oil contamination.
I've seen all this dozens of times .

The only contemporary injector system of the Djet which had a rising rate injection pressure was the Ljetronic as fitted to BMWS etc. It had a vacuum controlled pressure regulator which increased fuel pressure when acelerating.

* http://autospeed.com/cms/A_111327/article.html

BTW, the Volvo crowd fit a vacuum controlled pressure regulator when they convert from Djet to Megasquirt. I read the explanation but do not remember what it is exactly. Megasquirt seem to need it to work properly. On the other hand, the 450 SL Djet that has been converted by Gurunutkins seem to have retained the Djet regulator. Do you have an opinion on this?

Volvo,depending on which model mostly had Kjet injection which is a constant flow mechanical type . I am guessing but i think they may be turbo volvos they are using the variable regulator on. You need to have a rising rate of fuel pressure on some applications to prevent lean running at high pressure and engine speed. The Megasquirt can use either but again only constant pressure is required for the majority of applications. As long as there is sufficient Flow ,the pressure available is enough to supply fuel in a Djet. Even at full throttle,a lot of fuel is still returning to the tank,but on high consumption applications as on a race engine,then variable pressure regulators become necessary to prevent the lean condition I mentioned before.
The chances of Joe Average needing that on his driver are very remote.

You have a lot of threads & a lot of status updates. I've been away this whole weekend, can you specifically describe the miss/stumble? Only when warm? Only at certain RPMs? Have you checked the mechanical advance w/a timing gun and verified both idle and 3K RPM timing? Have you made sure the advance is not sticking? Have you tried plugging the vac retard to the distributor and setting to 7°BTDC instead of 5°ATDC w/vac? Have you tried bypassing the Pertronix (using stock ignition box/point setup) as a troubleshooting step?

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

You may have missed my point - The differential pressure does change even although the rail pressure is constant. With throttle plate closed at idle, you get an extra 9psi of differential pressure due to manifold vacuum which would push a LOT more fuel through the injector, if not compensated for!

I have no measurable leakage through my injectors - I checked them tonight - held full 30 psi for 30+min ( I have dash gauge that operates without starting pump). Some cars run the same injectors at higher pressures.

The article you referenced does not discuss 5-10% ethanol in our fuels or winter additives like butane, propane, isobutane that we have to stop freezing. These have changed the TVP (True vapor pressure) of fuels as compared with when our cars were designed. They make our fuels more volatile. (I think I said that before?) At 27 psig and 100degC, our fuel could boil and yet still be within spec.

I am sure you are trying to help. But we seem to be going in circles.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I have got rid of the miss/stumble. It was almost certainly caused by a worn set of trigger points. As the cam followers wear, the points stay closed for a longer period. When worn, they are also much more sensitive to how they are mounted. My spare set were in better shape.

The car is now running reasonably well. It always starts easily, even when hot. But it requires some pedal pumping to keep it going for the first few seconds. No leakdown of fuel though.

Regarding timing, I have usually used about 27degBTDC at 3000rpm without vac. But, I think this time I set it at TDC with vac at idle or thereabouts. I have been playing with timing a bit just to see how vacuum is affected. Car runs cool with higher advance, but am thinking it might be more efficient if retarded a bit? Maybe get better combustion? My CO levels are higher than they should be.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

i had a problem with rich mixture which was due mostly to triggerpoints staying closed too long.i removed the distributor and bent the contacts so they open for 110 degrees.that really helped.

Great that it worked! Replacing those points can be expensive!

I had one set that were so bad car would hardly run. I "adjusted" a second used set and they are still in car and working great. I was GIVEN a brand new set still in box, but see no reason to install them at present.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I tried this on my 3.5 with Megasquirt - installed a vacuum controlled pressure regulator set to run @ 28psi at atmospheric pressure. The car hated it as it produced fuel starvation and dramatic lean running under load (which makes no sense at all). I removed it and the car ran perfectly with the original pressure regulator. In theory it should guarantee stable fuel pressure under load (rather than fuel rail pressure reduction when all the injectors are open) and this in theory is better for MS because the computer algorithms assume stable fuel pressure.

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Alastair from South Australia
1971 280 SE 3.5
1981 300D
1980 300D

I am not sure if this is correct, but as I understand it, the D-Jet has two separate mixture controls. One is based on the MPS and this operates when car is under varying loads. The other is the idle control circuit in the ECU that does not use the MPS.

My guess is that the idle control circuit is designed to control the mixture when the injectors have vacuum on the discharge side (and therefore higher flow). No need to sense vacuum because it is at max (15-20"Hg). Switch on TPS tells ECU when engine is at idle (throttle plate closed).

Once throttle is even slightly opened, the MPS senses manifold pressure as it varies from say 20"-0" Hg and sends signal to ECU that controls the mixture accordingly.

I know almost nothing about MS, but if it is set up to work without compensating fuel pressure for manifold pressure, then I suppose it should be able to program it accordingly?

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I am guessing that the theoretical advantage of the vacuum adjusted fuel pressure regulator (which is tapped into the MPS vacuum line with a T piece), and the reason why they are common on turbo rice burners is because one of the base settings for the MS ECU is injector flow rate. As injector flow rate is a product of fuel pressure from the fuel pump, and is not controlled by the ECU, the vacuum adjusted regulator is supposed to ensure that the flow rate is increased in response to reductions in manifold pressure. When I set mine at 28 psi at atmospheric pressure, the fuel pressure dropped to 25 psi at idle which did not seem to affect the idle at all. However, for reasons that I still don't understand, it created a fuel restriction under load. It may have been simply because the SARD regulator can't cope with the gargantuan thirst of the 3.5, but I doubt it. Pic of the setup attached.

Attached Thumbnails

 

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Alastair from South Australia
1971 280 SE 3.5
1981 300D
1980 300D

Hum, that's weird. May be the adjustable FPR had a flow problem. I'm planning to use a Vanagon one.

Injector flow is a result of the difference in pressure across the injector. If manifold pressure is negative (as at idle), fuel pressure would have to be reduced to maintain the same injector flow rate. Maybe that is what you meant? Fuel rate back to tank would have to be increased by opening FPR.

You could run into hot start problems if fuel pressure dropped to 25psig at idle. It might be an idea to set atmospheric fuel pressure at something higher - say 33psig and then enter a correspondingly higher injector flow rate in the MS set up.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5