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Discussion Starter #1
I was hauling travel trailers 50 miles to a RV show for a dealer today. While pulling the trailers down the interstate, I drove about 70 mph, but when coming back empty for another load, I was running 75-78 mph. A number of times while coming back empty, for no reason the motor would start making a sharp, rapid, knocking sound similar to the noise when you tromp on it at low speed. EGT's were normal, temp okay, trans okay, no lights, and there was no change in speed or rpm. Edge EZ was in #1 position all the time. The noise would eventually go away (5-10 seconds) if you held a steady speed, but would stop immediately if you let off the gas even just a little. Fuel filter changed 1500 miles ago.........What's going on???
 

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If it sounds like when you tromp on it from a dig, it's the pilot injection cutting off. Completely normal.

Sounds like a loud rattle, right?
 

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its called your engine sounding like a diesel when all the fancy pilot injection shuts off
 

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what is the pilot injection??? mine does the exact same thing and dealer couldnt find a problem with it when i took it in
 

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Basically pilot injection injects a little fuel into the cylinders prior to the full injection to max the amount of power and make things run quieter. Above 2500 RPM's it can't keep up to itself and turns off.
 

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Thanks... Learned something NEW today! Knew the sound was normal. Didn't know a proper explanation. Now I do. Pilot injection.
 

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Basically pilot injection injects a little fuel into the cylinders prior to the full injection to max the amount of power and make things run quieter. Above 2500 RPM's it can't keep up to itself and turns off.
I actually can't find anything inside of the stock calibration that shuts the pilot injection off at a value of 2500 rpm. There is a parameter that shuts it off at 85 cubic mm / stroke. That is the volume of fuel that the pilot injection disables at. For anyone with EFI Live, the Parameter is B0749, and the description is: "
If the desired fuel rate for the main injection pulse is above this value then the pilot injection pulse will be disabled."

This log plot proves that you can be above 2500 rpm and still have pilot injection active. I'm at 2683, and still have 1.6 cubic mm of fuel from the pilot injection.


A second later, the fuel went above the threshold of 85 cubic mm/stroke and voila, the pilot injection shut off.

Myth debunked :cool:
 

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The original gm bulletin stated both rpm and fuel rate flow limits for cutoff. Reason for pilot injection cutoff was for epa emissions (which most here don't seem too concerned about) - guess it would be nice to raise the value higher.

With the data logging and capabilities of that programmer you have. I wonder where we will all be a year from now. I usually don't use profanity - but the data logging and control is some pretty cool sh*t.

jeff
 

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Discussion Starter #9
The wealth of information contained on this web site is just awe inspiring. I sure hope some of you guys are working in a field where you can put all this knowledge to use.

Thanks for the info on my "75 mph Rattle". At least when it does it, I'll know it isn't anything serious. Thanks again.......Roy
 

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Pilot injection - on a Duramax, about 2500 rpm; in an airplane, about 25,000 feet. ;)
 

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what is the pilot injection??? mine does the exact same thing and dealer couldnt find a problem with it when i took it in
In my LLY manual, there is a section in the diesel suppliment owners manual that explains pilot injection, knocking etc. and the conditions when pilot injection is on or off, what it sounds like etc. Basically under heavy load or about about 2500rpm there is no pilot injection and you hear the more "normal" diesel clatter. Pilot injection injects a very small amount of fuel to start combustion then adds fuel during the stroke to minimize the initial burn pulse and noise.
 

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Right. ABOUT 2500 rpm, BECAUSE of other factors, such as commanded fuel flow.
 
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