CM871 poor power and fuel econ

[Grounded Pilot]

Howdy y’all!!

I have an ongoing problem with the truck I drive. Power and fuel econ took a huge down turn a few thousand miles ago and I’ve been chasing problems with a mechanics help while on the road.

Fun facts:
Guy I drive for bought the truck in June ‘21. It was already d.p.f. deregulated and has a 550hp/1850tq tune. Both of which I have no idea who did the initial work. The shop we use doesn’t have a problem working on it. They did replace the ECM early ‘21 for the previous owner (who had the truck for a year, didn’t do the deregulate), So I’m assuming they did the latest flash of the ecm. Had good power to start with and made about 5-5.2mpg ave on paper. Then in the last 6,000-8,000 miles the mpg dropped to under 4 and power is terrible. Does not run very smooth either.

So far, we have replaced: (chronological order)
1: Cam (#1 intake lobe flat)
2: Fuel/timing actuators (blew the timing actuators apart in successive weeks. One fueling actuator was changed due to excessive play and the plate had scored the inside of the actuator)
3: Turbo lost an oil seal and was changed along with #6 injector due to failed cutout test.
4: last fueling actuator because the motor was running terrible in the cold and had a blue haze persistent from the exhaust at idle (did reduce the haze, made the motor slightly less erratic at idle)
5: MAP sensor and exhaust gas pressure sensor. (Mechanic suggested it after I asked about them, the exhaust sensor was broke off when I went to unplug it. Brought fuel mileage back up to reasonable levels but still .75gpm short of previous performance)
6: fuel cutoff solenoid. (Motor quit shutting off with the key. (Stopped at the shop and they ran a cutout and cylinder isolation test and tested the voltages and determined the solenoid had to be bad. Still won’t shut off and still runs like crap)

And that’s where I’m at now. I do a fair amount of the work because I don’t have time to wait for the shop to get the truck in. After running the tests on the ECM yesterday and changing the cutoff solenoid, our mechanic believes we have a hairline crack in the fuel pump/filter housing letting fuel go to wherever it wants. So, I’m heading to drop a load and then head home and change out the fuel housing.

My question is: Does that seem like a plausible explanation as to why the motor has so many issues??

Side note: the more I work on this truck the more I think it was neglected… a lot.

I have been sorting through all the info I can pertaining to the cm871 here since I discovered the forum a week or so back. I will also be cleaning up the EGR system while I am working on things. But my concern is that I do all this work and the motor still doesn’t perform because I didn’t get the feeling our mechanic was absolutely certain this would cure it. So I’m checking all the info I can find.

Thanks and sorry for the long post!!