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Was this issue happening before Diesel Doctor did the tune? I ask becasue we have been doing a lot of resets on the engines after he does his custom tunes on may trucks. All of them have had the same complaints. Good low end power with a top end drop out.
Do you have a fuel tester where you can test the fuel pressure? Were the check valves checked to make sure there are no issues?
There used to be issues on those older Detroits where the fuel plate behind the ECM was causing aeration. Bypass the fuel plate and see if you get your power back. Also put a clear line on the fuel line from the filter housing to the fuel tank line. Just a small jumper line to be able to see if there is aeration at higher RPMs. This can help rule out fuel tank pick up tube issues. On older vehicles, the pick up tubes will crack and allow air in which gives an aeration issue. With you not having smoke and such, I'm leaning to either a aeration issue or a check valve/regulator issue.
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