Iowa | Boy, that was hard to read.
First, Do you have a serial number of the engine?
Second, If I just go by the last 5 lines, It does somewhat sound like combustion gases are leaking by the injectors, where they seat on the "copper sleeves". It may be only doing this, while the engine in under load (like 6th gear) when cylinder pressures are higher(lugging the engine).
If it is a 3116, whether it uses mechanical or HEUI style injectors, they both have copper sleeves that the injectors "seat" on.
I ask for a serial number, because I want to know for sure, about the fuel system(Mechanical or HEUI). Yes, I know you mention HEUI in your story, But people get confused on this stuff, and I don't want to go down the wrong road, here.
Has your shop, every put a fuel return line "sight glass" on the vehicle, than gone for a test drive?
 (sight glass views.jpg)
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 sight glass views.jpg (45KB - 1005 downloads)
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cat 3116 big problems
