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I have a 2006 GMC C7500 with the 7.8l Duramax and an Allison 3500 RDS transmission. I pull a 40 gooseneck trailer about half of the time. The problem I'm having is when the engine is, under a load, it puts out enough black smoke to get me pulled over by the police. I can control it somewhat by paying VERY close attention to how I press the accelerator, but if I'm loaded down and going uphill, there is no way to stop it from smoking.

I had all the injectors changed in November of 2011. All filters were changed (they are and have always been changed when they are supposed to be). It made no difference. There has only been one time (since it started smoking) when it did not smoke. I was pulling my trailer with about 9k of equipment. From Houston TX to Beaumont TX it ran like a champ, but when I slowed down in Beaumont for heavy traffic it started smoking again. From reading other forums including this one my suspicion is that it has something to do with the engine not getting enough air.

Some other things I have noticed:
1. I can here the turbo when I accelerate.
2. Sometimes when I accelerate on an incline
it will not downshift unless I completely
remove my foot the press again. Then it will downshift.
3. Truck is equipped with an exhaust brake. When
I first got the truck using the exhaust brake would let out
a big puff of smoke when the brake was diseengaged. The
brake will still slow me down but it no longer puffs smoke
when disengaged.

This is my first time posting to any forum and my hopes are high. If there some additional info about my truck that I left out just ask. Thanks in advance.
 

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sounds like a boost leak for sure.
 

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I have a 2006 GMC C7500 with the 7.8l Duramax and an Allison 3500 RDS transmission. I pull a 40 gooseneck trailer about half of the time. The problem I'm having is when the engine is, under a load, it puts out enough black smoke to get me pulled over by the police. I can control it somewhat by paying VERY close attention to how I press the accelerator, but if I'm loaded down and going uphill, there is no way to stop it from smoking.

I had all the injectors changed in November of 2011. All filters were changed (they are and have always been changed when they are supposed to be). It made no difference. There has only been one time (since it started smoking) when it did not smoke. I was pulling my trailer with about 9k of equipment. From Houston TX to Beaumont TX it ran like a champ, but when I slowed down in Beaumont for heavy traffic it started smoking again. From reading other forums including this one my suspicion is that it has something to do with the engine not getting enough air.

Some other things I have noticed:
1. I can here the turbo when I accelerate.
2. Sometimes when I accelerate on an incline
it will not downshift unless I completely
remove my foot the press again. Then it will downshift.
3. Truck is equipped with an exhaust brake. When
I first got the truck using the exhaust brake would let out
a big puff of smoke when the brake was diseengaged. The
brake will still slow me down but it no longer puffs smoke
when disengaged.

This is my first time posting to any forum and my hopes are high. If there some additional info about my truck that I left out just ask. Thanks in advance.
I KNOW EXACTLY WHATS WRONG ...your leaking oil into your turbo. It will run forever like that and it will pass inspection but that is the problem.

Now you get to tell people a girl told you how to fix it...cause I'm female.
 

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Boost leak. Start by checking the couplings for the intercooler piping. Hopefully its just slipped out or a crack in a boot and an easy fix.
 
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