Fuel pressure at idle??

[JMBT]

CM2250, deman, @m*m^2, what measured fuel rail pressure are you running at low idle? Thanks

[Rawze]

That would all depend on where someone programmed it to if it is custom tuned. What is yours reading? and at what rpm?

[snailexpress]

Here is what i have
Engine warm to 185 F
Fuel pressure commanded 12779 psi
Fuel pressure measured 12759 psi
Actuator duty cycle 40
Fuel pressure sensor signal voltage 1.73
Engine RPM 649

[JMBT]

Thanks guys, Rawze: engine warm at idle 650 rpms, I am showing commanded 11900 and measured jumps from 9900 to 13300, it never settles on one value. How does that jive with what you know of my tune? Quick serve says if it fluctuates more then 1450psi at idle, replace the actuator.
Snail: thanks so much for the info. I think you may have been right, my actuator may be toast! Fricken Cummins! That actuator was brand new with the pump last February. Less then 100k ago,....bastards!
What do you guys think? Replace actuator and relief valve based on the fault trees for 3727 and 1911? I will test the pressure sensor in the morning when the rail pressure has fallen off, quickserve says it should be at 624psi key on engine off.

[Starlight]

If it falls of to the 624 mark then you have a leak. A healthy system will hold 6k psi overnight. Also 11k psi at idle seems accessive. 5-6k is more reasonable from my experience.

[Unilevers]

not on a custom tuned engine, it can be raised in the idle regions.

personally i would like to leave it at 40000psi and just run it all the time there, but the pump wont take it.

[Rawze]

Fuel rail pressure mostly determines fuel penetration into the cylinders. The higher and faster the fuel penetrates the cylinder, the more volatile it becomes and indirectly, slightly less timing advance is needed.

- From factory, the rail pressure in the newer engines is definitely a bit low (5-8k PSI). It is amplified by the higher compression ratios these engines have too. It does however coincide with the 50-70+% positioning of the VG turbo and the heavy EGR gas too. Bigger fuel droplets to go along with the longer burn, lower penetration, added inert gases all add up to meeting the emissions standard while advancing the timing to slightly get better economy. - Disconnect those cans and you can see it and smell the unburnt fuel it at idle.

- Running the engine with all the cans off it reveals that raising the rail pressure slightly increases penetration and improves efficiency,.. but injection timing must be changed to accommodate it. They kinda go hand in hand in an indirect relationship but not exactly. For example,... excess injection timing alone causes detonation, internal liner friction and vibration, excess combustion temperatures, sharp combustion peaks and other related problems. Excess fuel pressure will cause these same symptoms,.. but not as badly. Excess fuel penetration also adds the problems of fuel impingement, deposits, etching, and other factors that eventually take their toll in other ways.

The general consensus thus far that I have seen for those who have been doing custom tuning after a de-mandate is to raise the fuel rail pressure slightly at idle to reduce the unburnt fuel smell/quantity, but not so much as to cause penetration issues. The problem with this though is that it takes energy away from the flywheel to produce those pressures (the load placed by the pump onto the gear train), so any more pressure than necessary, and it takes away from overall engine efficiency. It is a delicate balance.

BTW: Older engines like the CM870/871's idle at about 23-28k PSI injection pressures and peak at about 32k injection pressures based on the information I have seen in the past. This is much higher than newer engines at idle, but then again, they do not have the compression ratio, nor the high injection spray angle, nor the finer spray that the newer engines have. It is like comparing apples and green-beans, they are two different things.

That is my own understanding of it anyways.

[JMBT]

So I checked it with insite from a cold start this morning. It was 22 degrees out and the truck had been off for about 12 hrs. The measured rail pressure still showed 2800 psi. When I started it up cold, at a low idle 650rpms, commanded was around 5800 and measured fluctuated by 1200psi. 5000-6200 After fully warmed up and running down the road a bit, I parked and checked again. We were back up to 11900 commanded at idle, with measured still fluctuating by about 3000psi. 9900-13000.

Sound ok for a custom tune? Or too low at idle, maybe accounting for smoking after idling to warm up?? What about the fluctuation at idle of 3000 psi? Cummins says more then 1450 to replace it, but would that be different with a custom tune??

[snailexpress]

(12-17-2017 )JMBT Wrote:  So I checked it with insite from a cold start this morning. It was 22 degrees out and the truck had been off for about 12 hrs. The measured rail pressure still showed 2800 psi. When I started it up cold, at a low idle 650rpms, commanded was around 5800 and measured fluctuated by 1200psi. 5000-6200 After fully warmed up and running down the road a bit, I parked and checked again. We were back up to 11900 commanded at idle, with measured still fluctuating by about 3000psi. 9900-13000.

Sound ok for a custom tune? Or too low at idle, maybe accounting for smoking after idling to warm up?? What about the fluctuation at idle of 3000 psi? Cummins says more then 1450 to replace it, but would that be different with a custom tune??

Definitely something is wrong in fuel system. If ECM commanded 11900 it should be close to this.
Start with fuel pressure sensor and it's wiring. also you can disconnect back flow tube from pressure relief valve and look if valve is leaking.