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Chris (aka-Moose)
Chris (aka-Moose), GMC Technician
Category: GMC
Satisfied Customers: 44503
Experience:  16 years of experience
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It is actually a GMC Duramax Diesel is this still the right

Customer Question

It is actually a GMC Duramax Diesel is this still the right place
JA: What is the model/year of your Chevy?
Customer: 2012 GMC Denali HD w/ Duramax Diesel Engine
JA: What size is your engine?
Customer: 6.6 Liter
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: Just diagnosing, it is repeatedly dying an producing a 1093 DTC code.
JA: Anything else you think the mechanic should know?
Customer: NO, other than it also goes into a limited power mode or Limp mode on occasion as well. Codes will clear and stay cleared from sometimes days but come back and then an entire series of events start happening like the motor dying, loosing power, etc.
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Chevy Mechanic about your situation and then connect you two.
Submitted: 5 months ago.
Category: GMC
Expert:  Chris (aka-Moose) replied 5 months ago.

Welcome, I'm Chris (aka Moose).

Have you done any steps so far to diagnose or repair this?

Customer: replied 5 months ago.
Diagnostics yes, no repairs. The Duramax 6.6 Liter Diesel engine will run fine until it gets hot, then it will normally go into a limp mode or reduced power mode. The primary codes are the 1093 DTC which is a fuel rail overpressure scenario. Also while running the tachometer will go to zero rpm for a few second then back up to normal running rpm. But the engine is still running. Which has to mean the tachometer is not sensing actual RPM but an electronically derived RPM. So the electronics think the engine is cutting out and falling to 0 rpm but It physically is still running. My personal opinion is the problem is electrical in nature due to this. The engine will also on occasion throw and error code of 1094 which Is a fuel rail under pressure error. When it sees this error code the engine dies. Again I suspect this is electrical in nature due to producing actual high and low fuel rail pressure if something like the CP4 fuel pump where going out or damaged would be a constant issue. Where electrical can be more anomalies or inconsistent vs. things like fuel pumps being physically damaged would show the same issues consistently. I have not put a gauge on the fuel rail to determine if it is actually in a high or low pressure state, but if the computer thought it was it would retard or increase the fuel flow in an attempt to compensate for the sensor feed back condition. I don't have access to GMC technical support bulletins so I can not determine if this series of codes, etc. correspond to a known condition of TSB denoted issue.
Expert:  Chris (aka-Moose) replied 5 months ago.

Thanks, ***** ***** through codes before symptoms and see where we are then.

Are you getting these codes from a scan tool or the instrument cluster?

There is a TSB for this code. https://dl.dropboxusercontent.com/u/90220402/12TSB.pdf

If your confused on what to do now. You have already been charged the full question value. It's now time to compensate the tech that helped you. The only way I can continue to work here and help customers is by getting rated. If your not pleased yet or plan not to rate me. Let me know so I can help more, close the post, or opt out. Please let me know if you are pleased with my help.

Thanks Moose.

Customer: replied 5 months ago.
The codes are being read by a scan tool they do not appear in the instrument cluster other than a check engine warning. When it goes into limp mode/reduced engine power this is displayed on the instrument cluster. The linked TSB doesn't appear to have any relevance. The entire truck is 100% stock with no modification. Are their any GM TSB on the 1093 or 1094 DTC codes. These would have the highest probability of relevance for things that could be possible causalities.
Expert:  Chris (aka-Moose) replied 5 months ago.

That was the only TSB for P1093. P1094 returns nothing. P0087 is pretty much the same code there is a TSB for it.

https://dl.dropboxusercontent.com/u/90220402/p0087.pdf

Measure return flow from the fuel pressure regulator FPR 2 port on the front LH fuel rail, (return hose removed and plugged). Place a hose on the port on the fuel rail, and place the other end into a container (fuel can). Start and run the engine. With engine at operating temperature, hold RPM at 1800 RPM, measure return flow from the FPR 2 for 15 seconds. If more than 10 ml, replace the fuel rail assembly on the LH side.

Expert:  Chris (aka-Moose) replied 5 months ago.

Hello again Chris here,
I am contacting you because the post has not been closed or rated. We have 7 days to get a post rated before it times out. When a post times out, that means its no longer on my active list (if you reply I never know it.) You have already paid the fees to get this question answered, but it appears your not pleased, or need more help.
Let me help some more. Is there anything else you feel I could offer so you would be pleased with my help. I have the option to call you if that would be better.

Thanks Chris

Customer: replied 5 months ago.
I contacted a GMC dealership and Service department and they pulled up numerous TSB on the 1093 and 1094 error codes. Unfortunately the 0087 has radically different meaning. IT points to Mechanical Failure of the CP4 Fuel Pump which is not the case. 1093 and 1094 can come up with mechanical problems with the CP4 but have a great number of electrical related things that are much higher propability given the symptoms. Unfortunately I had to pay for the Diagnostics at the dealership but now have a pretty clear indicator of the problem from the GMC TSB pulled up. Unfortunatly following the 0087 TSB would have lead to a very expensive diagnostics and repair process. I am assuming you do not have access to an official GM TSB system. the codes I denoted came up instantly in the dealership system. The problem following the actual GM TSB uncovered the problem and the solution which was nothing more than a shorted fuel pressure sensor wire. IT didn't even require replacing the Fuel Pressure Rail Sensor. Unfortunately you assistance wasn't very valuable and could have resulted in a very expensive diagnostics approach.
Expert:  Chris (aka-Moose) replied 5 months ago.

Glad you got it resolved.