P088 High Fuel Pressure Code

[LBZ]

Buddy is getting a P088 code on his stock ‘06 LBZ EC dually.
Hooked up his scan tool and his actual FRP is around 12k kpa, desired is around 6k kpa at idle where you want it.

FPR is at 64mA desired and actual, % was at 40% and no matter what you do with the pedal it doesn’t change and only revved up to 2000 rpm when I mashed the feeder.

Didn’t have my laptop to log anything or look at a stock tune, but does anyone have any ideas here?

I looked for a PTO input to the ECM and didn’t see anything, and I don’t even know if it has elevated idle as it doesn’t have steer wheel controls. It has an underhood VMAC air compressor but the only wires I could find were to a switch that engages the clutch. Not sure how/if the previous owner elevated the idle if at all.

 

[2004LB7]

Is the truck tuned? Does it have a lift pump?

Maybe the regulator is stuck. Or it's in limp mode

 

[LBZ]

Stock as stock.
Only other codes was for 2 glow plugs.
Limp mode could be a possibility but I did clear the codes once, but not while running.

 

[2004LB7]

I'd het your laptop so you could command different fuel pressure and see if the regulator responds

 

[LBZ]

Further questions to that if it doesn’t respond, Means it’s locked in limp mode or just doesn’t have communication with the FPR maybe? Both?
Is table B1016 supposed to be the current it max’s at in limp mode?
If so In my old stock LBZ tune it’s at 400 mA. In this truck it’s currently at 64mA according to the live data. Which should be WFO almost right?

 

[2004LB7]

Limp mode should be obvious with any codes (unless it's one of those weird ghost codes that doesn't show up but causing limp). My experience, high pressure can cause limp but limp doesn't cause high pressure. So if the only code you have is for the high pressure then I would lean towards stuck regulator.

B1016 is the current used when in limp mode. Since you say its running at 64 mA, then yeah it's trying to open it all the way. But if the pressure is high then why is it trying to open it more? Which makes me think it's not stuck and is opening.

Are you getting 64mA on the desired and actual?

Have you tried disconnecting the batteries for a few minutes to see if anything "resets"?

 

[LBZ]

Have a look at the screenshot I posted above. It has everything in it. That was at an idle. But yes 64 desired and actual for mA, but rail at 5800ish desired but actual double that. Seems weird. The mA thing is what’s throwing me. I’m missing something basic here…..

 

[2004LB7]

I see what you are saying. Definitely a head scratcher. I'd say try disconnecting the batteries and see if you see any changes

 

[LBZ]

I see what you are saying. Definitely a head scratcher. I'd say try disconnecting the batteries and see if you see any changes

Been done already when new batteries were installed. Problem was there before and after.

 

[2004LB7]

Well I'm out of ideas. If the tune is truly stock then the only thing left would be the ECM.

 

[Chevy1925]

Dont over think it.

jump under the hood and disconnect the FPR connector by the y-bridge and see if the rail goes to max. You could even give the regulator a few love taps and see if it starts working again. if it does, another tell tale sign it needs a regulator. 64 mA is virtually no current which means the regulator should be damn near wide open. The ecm may be trying to get it to move by letting spring pressure move it, could be a detail in the ecm gm applied when a regulator sticks. total guess but dont get stuck around that idea, look at the total picture.

if the pressure doesnt move, you got a stuck FPR. if it goes to max rail pressure and you plug it back in and it starts to follow commanded rail again, either you had a bad connection or you somehow unstuck the fpr. if it goes right back to 12k, you have a wiring or ECM issue. i wouldnt jump straight to the latter yet as i dont think you have tried what ive posted above?

 

[PureHybrid]

I'd say the regulator is stuck. What was the rail pressure when you revved it up? If the regulator is almost wide open, pressure should have spiked even higher with a little throttle input

 

[2004LB7]

I initially was thinking stuck regulator too, but why would the ECM not be trying to close it? It's only commanding 64mA. It should be commanding 1400mA or whatever the maximum is to try and close the valve. While it's possible like James suggested, that the ECM is on some algorithm that is trying to free the regulator, I've never seen or heard of such behavior.

The fact that it threw a high pressure code shows the ECM knows the pressure is higher then it wants so that should indicate that it doesn't want it higher. LBZ also indicated that the ECM would not respond to the DVT commands to raise or lower pressure. It's possible the regulator is stuck but why the odd behavior of the ECM?

 

[darkness]

I'd say the regulator is stuck. What was the rail pressure when you revved it up? If the regulator is almost wide open, pressure should have spiked even higher with a little throttle input

He said he rev’d it up with no change to pressure and only revs to 2000 rpm’s. Which is the limit in limp mode. Hopefully James is right as it does seem to be a stuck valve and the ecm is trying to correct it.
Dale, give that cruise set button a couple taps just to verify the high idle. Make sure cruise is on. But I don’t think that’s your problem.

 

[darkness]

I was also thinking maybe some chaffed wires between voltage and ground but not so much that it’s dead shorting? Just thinking out loud.

 

[Chevy1925]

I initially was thinking stuck regulator too, but why would the ECM not be trying to close it? It's only commanding 64mA. It should be commanding 1400mA or whatever the maximum is to try and close the valve. While it's possible like James suggested, that the ECM is on some algorithm that is trying to free the regulator, I've never seen or heard of such behavior.

The fact that it threw a high pressure code shows the ECM knows the pressure is higher then it wants so that should indicate that it doesn't want it higher. LBZ also indicated that the ECM would not respond to the DVT commands to raise or lower pressure. It's possible the regulator is stuck but why the odd behavior of the ECM?

my theory is based on driver capacity constraint in the ECM and thinking about it, ive got a little better theory. IIRC the lb7/lly could not take more than 2.5 amps. i believe the LBZ can take 5 amps but im not sure. cant remember but i think there are some older posts that state it. if its still limited to 2.5 amps and the ECM tries giving all that, you run the risk of killing the driver. I would think even still at 5 amps, GM doesnt want to throw a bunch of power at it trying to fix it. They would rather it throw a code and stop operating it till its fixed. i need to look up GM's reasons for setting this code and how it operates, we can deduce alot from there.

Instead, when it throws the code and does this limp mode, its allowing the regulator to hopefully have the spring pressure re-set the regulator and if it doesnt, its giving the truck the opportunity to have max rail but limited on rpm UNTIL the problem is fixed or the code is reset (even after the code is cleared, it would go right back to limp/CEL if its still stuck)

There might be a split second or little longer where once the code is cleared, those amperage controls are back to normal till the code resets and you MIGHT catch it on the scanner but youd have to be able to clear codes at the same time you scan data. a PICO scope would probably be better at this.

but again, this is all theory and not enough testing above has been done to confirm or deny it. Based on what ive been told from my engineer buddies over the years that work at different domestic car mfg, this is a common deal and makes sense in my brain.

 

[PureHybrid]

He said he rev’d it up with no change to pressure and only revs to 2000 rpm’s. Which is the limit in limp mode. Hopefully James is right as it does seem to be a stuck valve and the ecm is trying to correct it.
Dale, give that cruise set button a couple taps just to verify the high idle. Make sure cruise is on. But I don’t think that’s your problem.

The part he mentioned it only revved to 2000 rpm, only stated duty cycle and amps. Figured that was all he took note of

 

[2004LB7]

P0088

Diagnostic Instructions



Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. See: Testing and Inspection\Initial Inspection and Diagnostic Overview
Review Strategy Based Diagnosis for an overview of the diagnostic approach.
Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptors

DTC P0088 - Fuel Rail Pressure (FRP) Too High

Circuit/System Description

The fuel injection pump supplies high pressure fuel to the fuel injection rails, and then to the fuel injectors through high pressure pipes. The fuel rail pressure (FRP) sensor is a three wire sensor mounted in the right fuel injection rail. The FRP sensor uses a 5-volt reference circuit, a signal circuit, and a low reference circuit to monitor fuel rail pressure. This information is sent to the engine control module (ECM) to assist in the fueling of the engine.

Conditions for Running the DTC

DTCs P0087, P0192, P0193 are not set.
The engine is running.
Conditions for Setting the DTC

The actual FRP is more than 20 MPa (2,900 psi) above the desired FRP. OR
The actual FRP is more than 189 MPa (27,412 psi).
Action Taken When the DTC Sets

DTCs P0087 and P0088 are Type A DTCs.
The ECM commands the engine to operate in the Reduced Engine Power mode.
The message center displays Reduced Engine Power.
Conditions for Clearing the MIL/DTC

DTCs P0087 and P0088 are Type A DTCs.

Diagnostic Aids

This DTC will set if the electric fuel prime pump runs continuously during engine operation (van only). Refer to Fuel Pump Electrical Circuit Diagnosis. See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics
If the fuel system pressure is actually too high, a fuel knock will exist.
A sticking FRP regulator may cause this DTC to set.
Special Tools

CH-48027 Digital Pressure Gage

Circuit/System Verification

With engine running, observe the Actual Fuel Rail Pressure parameter with a scan tool. A normal reading would be approximately 40 MPa (5,802 psi) with the engine cold, and 30 MPa (4,351 psi) at operating temperature.
Engine running, observe the DTC information with a scan tool. DTCs P0087 or P0088 should not set.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
Circuit/System Testing

Start the engine and operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Observe the Actual Fuel Pressure parameter with a scan tool. The actual pressure should be 30 MPa (4,352 psi) with the engine idling at operating temperature.
Command the fuel pressure with a scan tool, incrementing through the entire fuel pressure range. The fuel pressure should increment and stabilize through the entire fuel pressure range with actual and desired fuel pressure within 5 MPa (725 psi) of each other.

If the pressure difference between the actual and desired pressure are more than 5 MPa (725 psi), test the solenoid supply voltage circuit and the solenoid control circuit for high resistance or a short to ground. If the circuits test normal, replace the FRP regulator.
If the pressure difference between the actual and desired pressure is less than 5 MPa (725 psi), refer to Diagnostic Aids.

 

[Chevy1925]

yeah didnt give as much as we'd want but for sure is in a limp mode and limited on rpm due to the code. Typa A DTC shows its a pretty high ranking code and kinds goes along with the theory but to know for sure, wed have to go deep than i think Dale can or really wants to lol.

Still points to FPR in gm's proceedure too assuming electrical checks out.

 

[2004LB7]

Yeah. It would be best to at least rule out a bad or stuck regulator. If that checks out then wires. After that there isn't too much left. It would be interesting to see if unplugging the pressure sensor shows the fuel pressure going to max just to make sure the ECM is being confused by a bad sensor too