i have a 03 2500 hd 2wd allison trans.
no modifications to eng.
when cruising on hiway at about 2500 rpm if i acellerate
it sounds like valve float in a gas eng. if i let off the
noise stops.......
injectors have been replaced by dealer because of rough
idle problem..no other work done and no codes show when i
asked dealer about the noise.......

You most likely are experencing the REAL diesel noise that all other diesel engines have at all speeds. The Duramax has Polit Ignition that sprays a small Amt. of fuel into the cylinders just before the full spray in order to start ingition and make the engine run much quiter.


When you get to 2500 or more RPM, the motor is turning over too fast to alow the Polit Ignition to work and the engine gets very loud until you get back down below 2500 RPM.


I have talked to Techs. that did not know the reason for the noise so don't expect them to have the answer to all of your questions. They just have not had enough experiance with the Duramax to know it all.Edited by: Durabill

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.

Info found @ http://dieselplace.com/forum/forum_posts.asp?TID=754&PN=2 , under [/url]
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