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Duramax Second Generation: 2004.5-2005 (LLY) Discuss the second generation (2004.5-2005 LLY) of the 6.6L Duramax diesel engine & associated components. 2006 LLY engine discussion belongs in Third Generation forum due to the differences of the 2006 LLY and the 2004.5-2005 LLY. Engine related discussion ONLY.

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Old 11-27-2018, 06:40 PM   #1 (permalink)
firedreamer
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Eng going into Limp mode, HELP

So, folks I need some help, we have a 2005 Silverado 3500 DRW LB 4x4 with the LLY Eng. We are full time RVers and have had no problems the last 2 yrs. towing a 15,000# worth of 5th wheel. Well I have had the truck go into what I think is the ďlimp modeĒ from what I can tell based on some quick internet searching 3 times the last month or so while towing. First time was after about an hour of towing over to the coast and on the last hill after starting up it happened until we got over (small hill, maybe 7 or 800 Ft elevation) and then it ran normal the last 2 miles to the rv park. The 2nd time we were not towing but again going up a slight incline but this time it only lasted about 5 min than back to normal driving. The 3rd time was about 4 or 5 min into towing and just as we started a very slight incline but it stayed in that for the whole 10-mile trip to go pump out. After being shut off while pumping it ran normal for 3 or 4 min on the trip back then back into limp. Ok the truck has run normal for well over 300 Ė 400 miles in-between these events at all speeds, normal sounding and feeling shifts, up or down inclines. It will run ok up to about 28mph and maybe 2500 to 2600 RPMs at about half throttle. If I try to give it more it starts to surge / buck. I canít get any more speed or RPMs but if I back off the gas it returns to fairly normal but at these limits. There has been no overrev like the tranny is slipping or anything. I had tranny fully serviced about 15,000 miles ago and its full of good-looking fluid. I changed the fuel filter and it looked good as well. I have looked in the eng compartment and find no obvious shorted wiring. I am going to take it to a shop I know and see if it showing any error codes but Iím not real hopeful thatís going to show much if anything. So, any Ides as to other things I can look at / for. I really canít afford an expensive repair at the min. We can sit where we are for the time being here in Oregon as the truck seems to do fine if Iím not towing. But Yuma would be a much warmer location... LOL
Any thoughts or advice are very much appreciated.

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Old 11-27-2018, 06:52 PM   #2 (permalink)
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You'll Need to check for any DTC codes first to know what direction to go..
Could be air in fuel system

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Old 11-27-2018, 07:33 PM   #3 (permalink)
heymccall
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Firstly, engine limp (reduced engine power) is an absolute cap on engine rpms. When in limp, the maximum engine rpms are 2000. And, it won't go away without a restart. If it goes over 2000rpm, when this occurs, and it clears without cycling the ignition, it isn't in limp mode.

Out of curiosity, what condition is the engine air filter?

When you bucking, and surging, is it pronounced (a lot of engine shake/ vibration)?

And, again, as stated above, there will most definitely be one or more fault codes (DTCs) stored.
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Old 11-27-2018, 07:33 PM   #4 (permalink)
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I am going to try to get it looked at and codes pulled this Fri, thanks. Its just hard for me to understand why it runs fine until a load is on it but thats was I spent a career as a firefighter and not a Auto Mechanic..... LOL Thanks again I am hoping its just a sensor somewhere
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Old 11-27-2018, 07:36 PM   #5 (permalink)
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I have several items of suspect on yours, but again, to save your brain power, we need the fault code(s).
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Old 11-27-2018, 07:38 PM   #6 (permalink)
firedreamer
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Well I may be wrong about the trip over to the coast .as to it running ok until i turned it off This last time it ran bad till i got to the dump site, it was turned off and 15 or 20 min later it started and ran good for a few min then back into running bad. as to the rpm limit im not 1005 sure of the top end its been a little bit.... and I'm old LOL
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Old 11-27-2018, 07:39 PM   #7 (permalink)
firedreamer
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Air filter replace at time of trans service , about 15,000 miles ago bit i will look at it in the morning
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Old 11-27-2018, 07:40 PM   #8 (permalink)
firedreamer
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I will come back to post any codes found in a day or 2 ..... thanks guys
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Old 11-27-2018, 07:44 PM   #9 (permalink)
firedreamer
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and yes seems like it had a lot of engine shake/vibration ... hard to describe as its such a subjective feeling i want to say "jerking" if I tried to give it more throttle but shakeing could be used as well
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Old 11-27-2018, 08:26 PM   #10 (permalink)
heymccall
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Then, I'd suspect that one of the injector connectors has lost its terminal tension. Very common on my '05 trucks (LLY engine series).

Many on here have used an ice pick to distort the terminals, and subsequently, the connector itself. I prefer to cut off the bad one (this is where the fault code comes in handy), and splice in a replacement connector w/ pigtail. The GM connector update kit is over $200, but, also includes brackets for strain relief. And, it only has 2 connectors, #2 & #7, which have proven to be the most problematic, but, with 4 LLY trucks, I've also seen issues with the connectors in other positions.
Aftermarket connectors are a whole lot less money.

And, if you haven't redressed the the FICM harness at the alternator bracket, now would be a good time to inspect, redress, that area of the harness.

Here, for the harness rub at alternator https://www.justanswer.com/gmc/7hg9e...ramax-lly.html

And, below is a injector connector.

Harness rub at the alternator or injector connector terminal tension are the two that come to mind, based on your symptoms, without having the DTCs.

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