Well in just the few years I have been in this hobby, I have seen a lot of people diagnose to failed ECUs on 3126s. More so than any other EFI system I have ever worked with. HEUI systems are fantastic genius level diesel tech, but I believe the 3126 was one of Cats first entries into EFI and I think mistakes were made, like the method they chose to retain memory with an unserviceable battery…
I am not recalling too many 3116 MUI core issues, I think someone needed a injector a while back, but most every issue I have helped with/recall circled back to ancillary systems, like primary fuel/air leaks, or run solenoid and power wiring. The MUI system in the 3116 is the pinnacle of over 100 years of active service and development. It is not as easy to modify, and 290 HP is probably closing in on its limit though.
You really dont need any special tools to work on it. the top end alignment doesnt require that kit, it just makes it easier to accomplish the task. Since tech has moved on, finding someone with the knowledge to figure out how to do that work is getting more difficult. I would rank an injector cup replacement as more difficult, but that is a task that could be potentially required on all these engines as they use the same fuel gallery and injector cooling scheme, so the tools and talent for that are still readily available. At their core, they are basically the same engine…
Diagnosing or upgrading the HEUI does require specific hardware, software, experience and permissions, as someone here just recently related. But those services are more readily available.
I hate the term high speed gears. Making 15-20K# of ex military vehicle go faster down the highway is not a good idea IMO. What they do accomplish for you is to make a normal highway speed more comfortable by lowering the cruise RPM closer to the peak torque point instead of riding on top of a cat screaming at 100% rated RPM… The second and possibly more important benefit is to lower the RPM on driveshafts that have pretty extreme angles.
Unless you throw a lot of HP and a different transmission at it, these trucks are never going to be fast uphill, deal with it
As always, my .02…