I hear you, but I know that my MAP sensor is 10% high at idle. It's also weird that it would be the only sensor that is showing targeted output instead of actual. MAP, IAT, MAF, Coolant, all show actual.

Plus why would torque suddenly show what I am forcing the ECU to see(900 bar). This is key on, engine off and it's reporting 13,000 psi in the rail. On a normal tune, same situation, it shows 0 until you start cranking the engine.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

Disconnect the rail pressure sensor with the engine running and see what torquepro shows.
On mine it will show the max value of 26000psi.

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U.S. Army Retired

That is interesting.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

Maybe there's a pull-up resistor on that input pin in the PCM so that if there's no input, it pulls that pin up to 5 volts or whatever.

For anyone not familiar with that term...
https://en.wikipedia.org/wiki/Pull-up_resistor
https://learn.sparkfun.com/tutorials/pull-up-resistors

I was wondering if this was maybe a safety thing. Like if it loses signal it defaults to the relief pressure and opens the valve to prevent damage. It would be easy enough to check, just tell the ECU that 5V = 1900 bar instead of 1800 and see if changes the output in torque.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

Pretty sure this is what's happening. I told the rail sensor map to be at 1900 bar above 4.5v and when I unplugged the sensor I got this:



You can see my MAP sensor is reporting 80 mbar higher than the barometric reading too. Makes me wonder if it reads 10% high across the board.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

Where does the barometric pressure reading come from? That little sensor on the lid of the air cleaner box?

Edit: Nevermind, service manual says there's a barometric pressure sensor built into the PCM.

In torquepro it comes from the Phones internal sensor. Torque cannot see the jeeps sensor. I can verify that my phone is accurate though because when I use Torque in my F150 it sees both the MAP sensor and barometric sensor from the truck and all three read the same, right around 800 mbars, which is what it should be at my altitude.

The jeep uses the PCM's sensor for tuning though.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

Thanks for investigating this. Totally changes my understanding of how the ecm manages rail pressure.

Does this mean that rail pressure can actually be increased to 1900 bar?
If so, care to write a 811 tune for me to test out?

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U.S. Army Retired

No. the sensor can only read to 1800 bar but you can go in and change the calibration map(what voltage = what pressure), which is what I did above. The sensor operates .5-4.5V, so if you changed the calibration to say 4.5V=1900 bar, then when the ECU thinks its at 1900 bar when its actually at 1800. This would actually do the reverse of the Powerpuck where it would reduce injection duration because it thinks its at a higher pressure than it is.

You cant exceed 1800 bar otherwise you would have a runaway. If you requested more pressure than 1800 bar, the sensor would max out, and then ECU would keep trying harder and harder to reach the requested pressure and never get there. It would close up the relief valve and the pressure would go through the roof.

I loaded up a tune today that runs at 1795 bar and so far its been fine. I gave it a little buffer so that i can overshoot slightly and the sensor wont already be maxed out. The Ecu knows that if it hits 1800 it should drop back down to 1795. I have bounced off of 1800 a couple of times and then it will drop back down.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

I dont see how runaway would be an issue.
It would only be at max pressure ar WOT.
Lift you foot and pressure goes back down.

So if you set it at 1700 bar, actual rail pressure would be 1900 bar, correct?
I dont see how this would be a problem.

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U.S. Army Retired

So there are two maps. There is the calibration maps which tells the ECU how to read the signal from the sensor. Then there is the desired rail pressure map which sets the target rail pressure for a given RPM and fuel quantity. You can set the calibration map to whatever you want, but the second the output of the sensor maxes out at 4.5V, the ACTUAL pressure will be 1800 bar. The sensors physical calibration doesnt change, only the ECU's interpretation of the sensors signal.

If you set the calibration to 1700 bar @ 4.5V, the actual rail pressure would be 1800. the ECU would never see more than 1700 bar since the sensor would never transmit more the 4.5V. If you set the desired rail pressure to 1800 bar in this situation then the ECU would be trying to get there, but because the sensor wouldnt be able to output more than 4.5V, it would think it never got there. Even if the rail pressure climbed well beyond 1800 bar the ECU would have no clue so it would keep trying to add more.

So yes, you could exceed 1800 bar but it would be totally uncontrolled.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs