I feel I have a very unique issue with My Ford f350 6.0 that no local expert can figure out. I have spent 1000's on mechanics just replacing stuff and I need some serious help. I have an 05 F350 with 10" lift runnng 47's. the truck stumbles and bucks, no back firing or coughing at 50 mph and up, RPM's between 1500-2500 most noticeably. Tire size is not a factor as I have gone back fourth with stock and there is no change. The truck only stumbles at 50 and up, 0-45 smooth. Tack is stable and does not jump around or flutter. the stumbling intensity does not change when RPMS go up and down when hitting a hill staying at 55. The heads have been done, I have the upgrade 07 Turbo and the after market EGR. Oil and filters changed regularly at 3,500. Truck does not stumble under heavy acceleration or steping into it. Turbo pressure maintains with load and is consistent with acceleration. No codes present at all, leak down on injectors did not show any issues. Im open for any guidance here.

 

I am familiar with all IC engines, all fuels, not versed with the 6.0 so this is all an educated guess, also not knowing whats been replaced or checked...
Light load stumble...usually = lean...(high side of Stoich). Maybe an air leak somewhere in the the intake track AFTER MAF air metering? At heavy load it wouldn't show up as the fuel is being dumped at it (rich or low side of Stoich). This also should rule out the turbo. If it was the problem you wouldn't get good full load response. Not sure if the waste gate is a factor or not? But if its doing something crazy when it should not be that will cause drivability problems. Whats the MAF IO saying? Can you simulate this problem while its hooked up to the service computer to read the MAF volts??? (Spec is usually 0.0-5.0 VDC/PWM).
Another idea would be, the IAT sensor (intake air temp) could be off screwing with the flow rate by giving the MAF loop a false temp feed back. Also one of the O2 sensors could be erratic or I have seen the wrong O2 sensor installed causing false or unrecognizable feed back.
A competent service tech with the right diagnostic tools and root cause analysis training should be able to ferret this out by reading all the IOs. My money is on a bad sensor or an air leak...or a combo of the two.

 

Thanks for the input. No issues with any of the above. Almost everything has been replaced even PCM and FICM. Injectors.ect.. just found a service update for my year truck. It appears this was an issue with my year and its requires a reprogramming and calibration of the PCM, FICM and TCM using WDS release B37.4 and higher. Must be done be ford. I have 5-6 symptom/issues called in in the service update. Im Pissed they didn't do this when I took it to them.. scheduled to take it in to different dealer tomorrow.. Hope this fixes it~