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This is my first Diesel (03 LB7), however I hear a noise, almost like knock at WOT. I hear it mostly on the highway, around 2500rpm.... Is this common? or is there something that I should look into?
 

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There is no more pilot injection about that rpm. Do a search on pilot injection and you will get more info. All of them do it. Completely normal.
 

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A customer who requests the fifth gear inhibit switch must understand fueling systems of diesels and the resulting noise that may be heard. This noise might be objectionable to some drivers. The following is provided in an attempt to enlighten the driver/operator of the vehicle.


Diesel engine fuel injection systems that use a common rail system, as the Duramax™ diesel does, may or may not have what is referred to as pilot fuel injection at the lower engine speeds. Pilot fuel injection is defined as a pre-injection of a small quantity of fuel 5 to 20 crankshaft degrees before the main fuel injection. This pre-injection softens the main injection pressure rise and reduces combustion noise. At engine speeds above 2600 RPM, and a fuel quantity supply greater than 35 cu mm, it is necessary to eliminate pilot injection to meet emission requirements. When driving at or above these speeds, main injection combustion noise may be noticeable. It is normal and not harmful to the engine.


When traveling in 5th gear, the pilot injection to main injection transition is typically in the 136 km/h (85 mph) range, however it may also be heard under heavy acceleration and when pulling loads up a hill during shifts at or above 2600 RPM. Customers that install the fifth gear inhibit switch may encounter the fuel knocking of main fuel injection at speeds around 107 km/h (67 mph) in fourth gear between 2600 and 2700 RPM. This sound is considered normal and is to be expected.

Found @ http://dieselplace.com/forum/forum_posts.asp?TID=754&PN=2
 

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Maybe I am deaf, but I don't notice a big increase in noise... matter of fact, I think the Duramax sounds good at wide open throttle



Dan
 

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StraitDiesel said:
Maybe I am deaf, but I don't notice a big increase in noise... matter of fact, I think the Duramax sounds good at wide open throttle



Dan
Same with me. I read numerous comments about this but I've never really noticed it myself. Must be because I keep it under 2000rpm
 

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Yeah I always keep it under 2000rpm
I was just saying it sounded good when I heard other people's Duramaxes' at WOT



DanEdited by: StraitDiesel
 

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I hear the sound whenever I am doing 85 to 90 and lightly let off the accelerate again. It is normal. At first it sounds like a timing issue, but reading the above post explains it well.
 

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yep, same for me. I've always noticed the noise, but didn't really worry about it cause it did it as soon as I got in the truck and has done it ever since I have owned the truck. So, I figured it was just a Diesel thang.


-Chad-
 
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