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That wouldn't happen to be about 2500 RPM would it?


It's the pilot injection turning off since it can't operate at high RPM, 2500 is about where it turns off. Then your Dmax will sound like any other diesel.
 

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I had read somewhere that the reason is that the pilot injection could not be employed above that rpm without violating federal emissions standards.


- jeff


Edited by: dmax lover
 

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Could be, but my understanding is that it's a physics thing. There just isn't time to execute two pulses of the fuel injection system above a certain RPM.
 

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From the TSB on installing OD lockout...


"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."
 
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