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I have a 2003 GMC Sierra 2500hd with LB7 and 97,000 miles, in the beginning of december i brought the truck to my dealer for tie rod and i complained of low power and no acceleration. When i was trying to accelerate out of intersections fast the engine would then lag around 2,500RPMS not shift out of gear and heavy black smoke would come out of the exaust until i eased up slightly on the pedal, then it will roughly shift to the next gear. The same condition appears when traveling on high speeds on a highway and attempt to accelerate or change lanes requiring more speed. Also when the black smoke appeared the SES light would instantly illuminate after a few seconds of smoke.

After bringing it to the dealer, i called the next day to see if it was done, they said the code came back as the glow plug relay, they explained 3 of the glow plugs were stuck in and they had to remove the whole head to drill them out. A week later after waiting on the rebuilding of the engine, i got the truck back the day of a snow storm. (I was unable to get up to speeds where the condition appears) As i drove during the day i began hearing a pinging noise at idleing then while going 35 the oil pressure gage started bouncing like crazy and the engine began knocking!!!! It died on the side of the road and had to be towed back to the dealer 100 miles after being serviced. This time the tech introduced himself he is a Duramax Specialist and seemed to know what he was talking about as it turns out the valve springs went causing the ping and knocks.

Now into the first week of January i now have my vehicle back, no more pings or knocks, but i now have the same exact condition when trying to accelerate hard. But yet i have no codes now that glow plugs have been replaced. Does anyone know what would trigger this and also i have never known glow plugs to be the reason for loss of power on an engine thats already warm.???? i also suspect this could be an electronic transmission problem, 5 or 6 times i have gotten a RANGE SHIFT INHIBITED message when first starting in the morning, i have no issues with gears slipping, only shifting up when accelerating highly.
The tech also explained the glow plug light is caused by a relay that comes on, and the plugs are not a necesity (as i never had a problem starting in the cold prior to replacing)
 

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Something liks this happened to a guy I know on his 01 3500. It was a real dog and smoked like a train, it would also trip a code when he tried to pass or just accelorate hard. He took it to two different dealers, one replaced the wastegate with zero result, the other tried to sell him a new turbo for an outrageous amount. One day he and I were looking it over as he explained his frustration when he grabbed the aluminium tube going out of the turbo into the intercooler and noticed that the steal fuel line that goes into the fuel filter had rubbed a dime sized hole through the tube! He patched the hole and the problem was fixed. Now it never smokes, or throws a code and has a lot more power. He says it's like a whole new truck. The funny thing is that the orignal owner said it did that most of the time he had it too. Now I'm not saying that this is what going on with yours, but it's worth a look. It's been my expierance that dealers often overlook the small, cheap, easy to fix stuff and go right for the technical high price jobs.
 

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Loss of boost would cause that condition, as outlined above^^.

If you are getting the range shift message, that also means the NSBU switch on the tranny is acting up.

Where are you located?
 

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i've had a low power, black smoke condition with my 01 on several occasions and every time it was fixed at the dealer, the maf sensor (the one on the air filter box) was at fault every time. hope that helps you. i would try taking that sensor out and cleaning it with alcahol or they have a spray can that is specially made for mass/maf sensors. if you have a k&n airfilter you definitly have alot of oil on that sensor.
 
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