04 Freightliner Coronado c-15 mbn cat switched to 6nz, having fluttering issues

I bought a 2004 Coronado equipped wth a c-15 500hp mbn eaton fuller 18spd 3:58 rears. I dealt with the lousy performance and fuel mileage for the past few years until May this year when I decided to convert the ECM to a 6NZ. Upon installation the timing was set to 6NZ specs, boost sensor was replaced, wastegate is diconnected, running 5232 injector settings and currently flashed the ecm set at 550hp with a file from 2008. With that said, I've got all the power in the world on takeoff and up until 7th gear direct. When splitting the gear up and upshifting it seems the engine de-rates and flutters for a few seconds before straightening out and regaining power. I'm searching for suggestions of what we're missing as we've tried the 6999 injector codes which made it sluggish on takeoff, tested the injectors (all check out fine), did valve and jake adjustment. The positive is: I've got the power of a 6NZ, and I went from 5mpg to just over 6mpg's. I do not haul heavy, maybe 20,000 in a 53' dry van so I believe I should be capable of getting more fuel mileage, but, my main goal now is to try and tackle what could be causing this awful fluttering in the last few gears. Thank you in advance for any helpful suggestions!

 

Thank you for the suggestion! I picked up an upgraded fuel spring kit from Cat this morning, will install tonight when I get back to my truck, I will post the outcome.

 

Not exactly sure at the moment. The last time the mechanic and I took it up the road while he was plugged into the ECM port it was showing about 34lbs. Since then we reflashed the ecm with a program from 2008 and the truck woke right up. The only issue now is when upshifting the last few gears there is a flutter for a few seconds before it straightens out. On takeoff and the rest of the gears boost is great, and power is there, it's just the last three or four gears. I did purchase the upgraded spring kit for the fuel pump, however, I have the older style fuel pump with the 5/8 hollowed out plug in which a much smaller diameter spring sits in. Earlier I shimmed it with two small washers to put more added pressure on that spring for better fuel pressure, it seemed to help take away some of the flutter, but not all. Tonight I will add two more and hopefully it will cure it. I don't have a boost or fuel pressure gauge in the dash so I am going by feel, trying to be conservative about it so I don't create too much fuel pressure.

 

After changin the injector wiring harness, shimming the fuel pressure spring, I found out I had been using the wrong water separator filter, put the correct one on and cleared up all the fluttering issues! Thanks to everyone for the input!