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Duramax Third Generation: 2006-2007.5 (LBZ & LLY) Discuss the third generation (2006-2007.5 LBZ & LLY) of the 6.6L Duramax diesel engine & associated components. 2006 LLY engine discussion also belongs in this forum due to the similarities of the 2006 LBZ and the 2006 LLY. Engine related discussion ONLY.

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Old 10-13-2011, 05:35 PM   #1 (permalink)
steverado
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Intermittent idle issue and extremely high fuel pressure at idle

Looking for some insight in to this problem w/ my LBZ. I've had it to the dealer, and their solution was a new ECM...which seemed to be the fix, until yesterday.

I've had an intermittent drivability issue with the truck. Here it is:

Upon start up, my fuel pressure gauge will read 15,000psi, and the idle will be up and down between 700rpm and 1000rpm. Its very rough, and sounds almost as if its hydrolocking a cylinder...a loud clanging, if you will. When I depress the accelerator pedal, the RPM's drop down as does the fuel pressure, but the engine drops to around 500rpm. If I try and drive, its definitely in some sort of reduced power mode, and I can't get it up over about 7mph.

There is no set time in which this happens, wether it be at operating temp, or cold start. Sometimes after I shut it down, and re-start after a few minutes, the issue is gone. Other times, it is still there.

SES light is on, but I don't recal the code at this time.

Any insight would be greatly appreciated. Thanks in advance.

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Last edited by steverado; 10-13-2011 at 05:38 PM.
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Old 10-13-2011, 09:00 PM   #2 (permalink)
dmtcm
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Sounds like you have a FPR sticking intermittently. Take a look at the fuel return ratings
and your injector balance ratings. If your balance rates are going high or low (positive or negative numbers) when the rail pressure goes high, that tells me your ECM is trying to compensate for the change in rail pressure which is normal for the ECM to do that. You should see numbers around -2.6 to 2.6 when it occurs. If the numbers don't go higher or lower than that, the injectors are fine. they should return to normal when the rail pressure comes back down to the desired pressure. Return pressure needs to vary with the changing rail pressure.
Knowing what the code is that you have will help figure out what's going on.
The rail pressure is getting too high for the injectors to be able to open properly. The knocking noise is the injectors struggling to open and the cylinders becoming too lean and creating engine knock.

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Old 10-13-2011, 09:19 PM   #3 (permalink)
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Quote:
Originally Posted by dmtcm View Post
Sounds like you have a FPR sticking intermittently. Take a look at the fuel return ratings
and your injector balance ratings. If your balance rates are going high or low (positive or negative numbers) when the rail pressure goes high, that tells me your ECM is trying to compensate for the change in rail pressure which is normal for the ECM to do that. You should see numbers around -2.6 to 2.6 when it occurs. If the numbers don't go higher or lower than that, the injectors are fine. they should return to normal when the rail pressure comes back down to the desired pressure. Return pressure needs to vary with the changing rail pressure.
Knowing what the code is that you have will help figure out what's going on.
The rail pressure is getting too high for the injectors to be able to open properly. The knocking noise is the injectors struggling to open and the cylinders becoming too lean and creating engine knock.
Thanks for your insight. We checked balance rates on the Tech2 when I had it in last time for the ECM. Didn't see anything +/- 1.9 w/ balance rates.

Im just having a hell of a time duplicating the issue on demand, so servicing it under GM Certified warranty is challenging. My technician is my best friend, so he pushes tech assistance all he can...but they're tight asses. Ill get the code tomorrow, but im pretty sure it was "LOW FUEL RAIL PRESSURE."
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Last edited by steverado; 10-13-2011 at 09:46 PM. Reason: spelling
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Old 10-17-2011, 08:53 AM   #4 (permalink)
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Looks like I am getting a new fuel pump/regulator and possibly 8 new injectors under warranty. Guess I'll have to see how this works out on Wednesday...
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