(SOLVED) Weird fuel pressure symptoms

PressPause

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Apr 30, 2022
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Alrighty here goes nothing…
My idle fuel pressure is at 9,000 psi I have tried commanding it higher or lower with my tech 2 to no avail.
I disconnect the FPR no change. If I disconnect the Fuel Rail Pressure Sensor I am then able to command a change of pressure with my tech 2.

2005 GMC Topkick 4500
Stock no mods
 

PressPause

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Apr 30, 2022
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You can hear it in the attached YouTube video
The sound of the motor and the quality of the idle changes as the desired pressure is adjusted.
how can you tell if it is actually responding if the pressure sensor is unplugged?
 

PureHybrid

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Feb 15, 2012
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I've only unplugged my rail sensor while it was running one time on purpose, but the engine died. I think you have another issue going on
 

dndj

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Apr 13, 2019
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Any codes?

Since your test of the commanded FPR seems to be having an effect on the engine, if you get really stuck I would leave +5/gnd only connected to the Fuel Rail Pressure Sensor. Break the signal wire between the sensor and the ECM and monitor what the sensor is reading while the apparent pressure change from the FPR is happening. Fuel rail pressure sensor signal should vary between 0.5v and 4.8v.
 

PressPause

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Apr 30, 2022
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My only code is P0089
With my sensor plugged in the idle psi fluctuates between 8000-9000 and cannot be modified with the tech 2
Any codes?

Since your test of the commanded FPR seems to be having an effect on the engine, if you get really stuck I would leave +5/gnd only connected to the Fuel Rail Pressure Sensor. Break the signal wire between the sensor and the ECM and monitor what the sensor is reading while the apparent pressure change from the FPR is happening. Fuel rail pressure sensor signal should vary between 0.5v and 4.8v.
 

dndj

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P0089: "The engine control module (ECM) uses commanded fuel pump flow to determine a desired fuel rail pressure (FRP). The actual fuel pressure is
monitored using the FRP sensor. If the FRP sensor indicates a pressure more than 20 MPa greater than desired, DTC P0089 will set."

If you've seen the measured (Actual) fuel rail pressure on your Tech2 moving around under various conditions, that would say it's probably not the fuel rail pressure sensor. Always a good idea to check ECM to FPR wiring as well for resistive connections/shorts/grounds. If this checks out, seems like the FPR needs replacement. Sure the plunger in the regulator doesn't appear to be totally stuck, but it still may not be able to close off properly.
 
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PressPause

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alright so I’m pretty dumb… my diagnosis process was even worse. For the first time I had both the sensor and the fpr unplugged and the pressure was still able to be adjusted… the FPR is just probably stuck.

P0089: "The engine control module (ECM) uses commanded fuel pump flow to determine a desired fuel rail pressure (FRP). The actual fuel pressure is
monitored using the FRP sensor. If the FRP sensor indicates a pressure more than 20 MPa greater than desired, DTC P0089 will set."

If you've seen the measured (Actual) fuel rail pressure on your Tech2 moving around under various conditions, that would say it's probably not the fuel rail pressure sensor. Always a good idea to check ECM to FPR wiring as well for resistive connections/shorts/grounds. If this checks out, seems like the FPR needs replacement. Sure the plunger in the regulator doesn't appear to be totally stuck, but it still may not be able to close off properly.
 

Bdsankey

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alright so I’m pretty dumb… my diagnosis process was even worse. For the first time I had both the sensor and the fpr unplugged and the pressure was still able to be adjusted… the FPR is just probably stuck.
The bolded above simply is not possible. The ECM may have "allowed" the change BUT nothing was actually happening to the pressure with the FPR unplugged.
 

PressPause

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Apr 30, 2022
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That’s exactly what I was thinking. But there must be another variable that’s being controlled fuel wise in order to change the tone of the engine while the pressure is being modified.

When I first played around with my tech 2 with another Duramax I played around with the fuel pressure and when I turned it up it sounded like an older diesel so I always correlated the pressure to the sound.
What’s weird is that the “pressure” of the engine was only able to be commanded with the sensor off which is why it sent my diagnosis in the other direction.
I was still able to alter the “pressure” with both unplugged like it did in the video. if it was plugged in the quality of the idle did not change with the tech 2.
So while it’s not possible something was in fact changing.
The bolded above simply is not possible. The ECM may have "allowed" the change BUT nothing was actually happening to the pressure with the FPR unplugged.
 
Last edited:

S Phinney

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Aug 15, 2008
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0089 indicates a regulator issue. Either some debris is causing it to stick or not move properly or the electrical circuit is bad in it. It’s a electro magnet basically.


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2004LB7

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Dec 15, 2010
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if you disconnect the pressure sensor shouldn't the pressure reading go to max? thus causing the ECM to command max current to the regulator to try and pull the pressure down? how was it only showing 9k with the sensor unplugged? short in the wire?
 

S Phinney

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Aug 15, 2008
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It should want to but out of all vehicles kodiqk trucks have some weird crap happen with fueling that you just don’t see on normal trucks.


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PressPause

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Apr 30, 2022
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Oh it Showed max pressure with it unplugged but my diagnosis method ended up tricking me.
The pressure regulator was stuck at 9000psi.
With the pressure sensor disconnected my tech2 gave me the illusion that I was controlling the “pressure” but it was controlling something else that was changing the idle quality when modifying fuel rail pressure.
(If anyone has any insight on what was being controlled with both the fpr frps unplugged that would be awesome)
if you disconnect the pressure sensor shouldn't the pressure reading go to max? thus causing the ECM to command max current to the regulator to try and pull the pressure down? how was it only showing 9k with the sensor unplugged? short in the wire?
 

PressPause

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Apr 30, 2022
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Anyways the problem was the FPR… with the FPR pulled out I noticed when plugged in with the ignition on it wasn’t making a “buzzing” sound that good FPR would make.

I would like to make a special thanks to my viewers for staying with me on this dramatic/mystery event.
That concludes this episode of “How To Diagnose A Duramax… The Long Way”.
Now to the P0237 I’m changing out the map sensor.