I have been chasing a lean condition and have been looking into the fuel pressure. This is a 4.2L with the returnless type system and the electronic regulator has always showed 55psi, give or take a little.

Thing is, in looking through JTIS I noticed today that the fuel system pressure specification is supposed to be 3.8 bar (55psi) for vehicles without supercharger and 5bar (72psi) for vehicles with superchargers.

Is that right? Should it be holding a 5bar differential to manifold pressure? Or am I misunderstanding what JTIS means?

fuel_spec.pdf

 

To be honest, I can't really remember. From some documents I saw it was 65 psi (thats what I am running them), but I have never read a JTIS document.

I can vaguely also remember seeing upper 55 psi from early recordings (which I don’t have any more unfortunately).

One of the reasons the 4.2 injectors runs at higher pressures is because of the number of finer holes from which it sprays (12 iirc) for better fuel atomization.

Anyone with a 4.2 car can check with an ordinary odbii meter what the fuel pressure is, so I hope some will post theirs soon here.

 

I was hoping someone out there with a 4.2 could verify.

I suspect that you are right and 55 is the correct pressure. It is pretty clear that the ECU is actively controlling to 55 as a setpoint, and the only way that it could be wrong and still doing that, is if the ECU were programmed incorrectly, which is pretty unlikely.

I could be that 72psi in JTIS means the MAX pressure when measured against a room reference. If you connected an external gauge you would see 72psi when you had 17lbs of boost, if the ECU were maintaining a 55psi differential to the manifold. That seems a funny way to document it, though.

If it does turn out that 55psi is the stock differential pressure then a twin screw is never going to work well on a 4.2 without doing something to the fuel system to get the pressure up (or going to bigger injectors). 55psi will have about a 10% fueling disadvantage compared to the 65psi you are running.

As a test, I pulled the manifold reference off of the pressure regulator and went for a drive. At cruise condition this would add about 6 or 7psi to the differential fuel pressure since the manifold is somewhere around 7psi and the ambient reference is ~14.

That dropped the cumulative fuel trims (STFT + LTFT) about 6% and also seemed to cure the stalling problem. I back calculated the improvement and if I could get it up to a consistent 65psi, that should put the total fuel trims at about +10%.

If I can figure out how to attach an external gauge, then I don't think it will be too hard to fake out the sensor and get it to control to a higher pressure.

 

That's incorrect, I have run with the stock 4.0 injectors before and stock pressure of 43 psi. so even less potential than 4.2 injectors at 55psi.

 

When you ran that setup, was that with the 2.6L, the 4.2 block, the big intake elbow and the larger MAF that you have now? Or was it an earlier configuration?

I have the pressure adjustment circuit made up, just need to make some baseline measurements so I can set the initial fudge factor.

 

I was thinking about this some more this morning, but beware, it is before I have had enough coffee so we'll see if the logic holds up.

My understanding is that setup with the big KB, the free flow intake, larger MAF, running at 65psi pressure should give near zero fuel trims.

All else being equal, decreasing fuel pressure to 55psi would be about a 10% increase in valve open time to deliver the same amount of liquid, so +10% FT.

If you again decrease pressure to 43psi then it would be another 10% increase in duty cycle, or +20% FT to get the same flow.

I'm not saying that it wouldn't work, just that it is marginal. And if you start off with a car that is already running positive adjustments for one reason or another, and then make all of the modifications, it is plausible that the necessary fueling adaptation would fall out of range.

 

Yes, wouldn't have said it otherwise. The 4.2 injectors you have flow already about 10% more then the 4.0 ones at stock pressures.

 

With the stock pressure it should be near zero fuel trims, if you increase the pressure on your system, you would get negative trims. There can be some deviations though, at least for the 4.0 cars, but all in all it stays withing the limits.

Am still not 100% sure what the stock pressure is, I have also seen documents with 4.5 bar pressure (which I took as stock). So lets see what others say with the 4.2 XKR what they have.

 

I doubt this helps your query too much and await the same answer to your question from others, but I have been informed to set a static pressure of 3.5bar for the 4.2 and use a non rising rate FPR with 3 way valve with a return valve back to tank for the excess fuel.

Of course this is for an application not installed in the standard car or using the standard ECU.

 

 

Hope the above helps, what do you make of it?

 

On my Torque obd2 program, fuel pressure is 54-55psi

 

Thanks. What I make of that is the fuel pressure is 55psi + 15lbs of boost, which looks like 70psi (4.8 bar) against atmosphere.


Thanks for confirming the reading.