I have asked this question before, however have never got a good answer. Every once in a while when I put my foot into it to pass someone I get a noise a like two geers grinding from the engine. I let off and it slowly goes away. What does turbo bark sound like? I have the LLY with the Edge and Attitude. It has done it a 1/2 dozen times now. It makes me a little worried!

Do a search on "pilot injection" and see if that answers your question.

Yup, definetly pilot injection shutting off. Its supposed to shut off when the engine reaches ~2500 RPM, but sometimes the computer shuts it off for some other reason, and the engine really starts to clatter. Its not bad, it just means it will be louder until pilot injection kicks back in.

Basically pilot injection is when the piston is at TDC, the computer commands a "pre-injection" dosage of fuel. The injector sprays just a little bit of fuel in to start the burn. Once this "pre-combustion" burn is started, it dumps the full load of fuel in for that stroke. It basically allows for smoother, "ramped up" combustion, rather than just exploding "violently" all the fuel at once. When the combustion is smoothed out like this, it makes the engine A LOT quieter. However, when the RPMs get above ~2500, the piston is going so fast and the cycles are happening so quickly that our pea-brain 16 bit ECM's cant accuratly measure out and inject these "pre-ignition" shots of fuel in time before the piston begins its way down the cylinder. So it just shuts it off and reverts to "normal diesel injection". And in turn, starts to hammer like a "normal" diesel. In our trucks it happends around 80 mph or so, but in the Kodiak's and TopKick's, guys complain about the "injectors really getting loud" around 65 mph...I would assume thats just becuase the medium dutie's are geared much lower, so ~2500 rpm happends at 65 mph rather than ~80 mph.

----ben

Oh yeah, and any chips or tuners will make pilot injection act kinda funky and make it turn off at random times...