LB7: Need some serious help diagnosing fpr/rail issue

duramax hd

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Feb 27, 2009
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Hey guys been a while since I've messed around with my truck but it's time I dive back and put these issues to bed because I am at a loss on what to look for.

A little back history, after getting out of my mess of a used built motor I bought, I found a motor from a member on here out of a running wrecked truck. After putting the motor in, all was well for a short period of time. Rail pressure was hanging up and taking a while to come back to desired when slowing down so I attributed it to a bad FPR. I had a brand new from GM LBZ pump in a box that I had on my built motor so I decided to swap it out. Tune was reflashed to work with the LBZ pump, but I have max rail pressure at idle now(171mpa) demand is correct though at 38mpa. I thought the wiring could have been damaged so I ohm'd out the wire to the bale connector and then to the ECM and it seems to be good. I checked voltage on the two wires (purple/black and yellow) and have 5v on the purple wire and .3 volts on the yellow. I have some codes which I've been trying figure out if they are related and could cause the ECM to be outputting the wrong voltage.

P0090
P0089
P0108
P0380
P0540

I know 380 and 108 and glow plug and air intake heater codes that in not concerned with as I have bigger issues and I don't believe those are tied to the rail pressure issues.

ECM has been verified good in another truck with an LBZ pump. The map and baro I believe are hooked up properly. I swapped rail pressure sensor with another I had and get the same results. Cleaned MAF with no change. Is there something in missing?? What other reference signals control what is outputed by the ECM that I can check? What has my very confused is that demand rail pressure is right on but actual is not following. So what the hell tells the ECM to output the right voltage? Any help is greatly appreciated
 

Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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hey Kris!! long time no see!

unplug your lift pump and see if your rail issues fix them selves
 

Awenta

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Sep 28, 2014
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Bad rail pressure sensor or wiring?

Sent from my SM-N910V using Tapatalk
 

duramax hd

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Feb 27, 2009
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:hug:
hey Kris!! long time no see!

unplug your lift pump and see if your rail issues fix them selves

Hey James yea it's been a while. Congrats on the little one! Still on here lurking every day but had been pretty discourage with my truck for a while because I can't seem to figure this thing out.

Tried unplugging the lift pump with no changes to rail pressure. The regulator isn't getting any voltage on the signal wire so it can't clamp the regulator down
 

duramax hd

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Feb 27, 2009
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Bad rail pressure sensor or wiring?

Sent from my SM-N910V using Tapatalk

It may be bad working somewhere but I don't think it's wiring to the FPR though as I don't have the proper voltage coming out of the ECM. Tried 2 rail pressure sensors too
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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:hug:

Hey James yea it's been a while. Congrats on the little one! Still on here lurking every day but had been pretty discourage with my truck for a while because I can't seem to figure this thing out.

Tried unplugging the lift pump with no changes to rail pressure. The regulator isn't getting any voltage on the signal wire so it can't clamp the regulator down

thanks buddy! :hug:

have you tried swapping ecms with anyone?
 

duramax hd

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Does anyone know what the correct voltage should be on the 2 wires that go to the FPR? Also does anyone know what else tells the ECM to send that voltage? Map sensor/baro sensor/fuel temp sensor/MAF sensor are the ones I'm guessing. Can one of these sensors be sending the wrong signal to the ECM causing it to not output proper voltage to FPR?
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
21,637
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Phoenix Az
Does anyone know what the correct voltage should be on the 2 wires that go to the FPR? Also does anyone know what else tells the ECM to send that voltage? Map sensor/baro sensor/fuel temp sensor/MAF sensor are the ones I'm guessing. Can one of these sensors be sending the wrong signal to the ECM causing it to not output proper voltage to FPR?

check your coolant temp sensor reading and baro. off the top of my head, those i think are the only modifier tables in a LB7 to affect it.
 

2004LB7

Well-known member
Dec 15, 2010
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Does anyone know what the correct voltage should be on the 2 wires that go to the FPR? Also does anyone know what else tells the ECM to send that voltage? Map sensor/baro sensor/fuel temp sensor/MAF sensor are the ones I'm guessing. Can one of these sensors be sending the wrong signal to the ECM causing it to not output proper voltage to FPR?

the FPR voltage is actually a pulse width modulated 12 volt signal. reading it with a regular voltmeter will give you weird/incorrect readings.

Air intake temp, Engine coolant temp, Baro pressure, are all used by the ECM to make adjustments to the desired fuel pressure but none of them are drastic enough to make the fuel pressure go wildly out

the two places you should concentrate on is the pressure sensor and regulator wiring. both of those can cause what you are describing

Jason
 

2004LB7

Well-known member
Dec 15, 2010
6,925
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here are the schematics for the pressure sensor and pressure regulator
 

wilsondiesel

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Nov 11, 2015
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Call and ask for Dan at PureFlow Technologies 573-635-0555 he had one do the same exact thing I helped him diagnose over the phone. Check the pins on the C2 ecm connector, pin 77 and 42 If I remember correctly he replaced those pins.