Qsx15 turbo wastegate control valve?

[Moffat]

I’ve got a qsx15 cm570 s/n79165811 cpl8761in a case tractor stx480 with some issues I’ve been struggling with for a few years. It’s low on power and not smooth power curve. Will hang around25psi boost for most of the time but over a certain load it’s like it fuels hard and hits 35ish. More noticeable powerband issue when rpm’s are higher. I’ve set valves and injectors. Set base timing to 4 and 6 as per spec. replaced all fuel and metering actuators(fixed a rough cold start. Replaced Intake pressure and temperature multisensor. Replaced Barometric pressure sensor. Cleaned hexagon fuel screen and smaller fuel screen above hex screen. Blown back in fuel lines. Changed wastegate actuator(diaphragm was leaking). Had a retune to set it to an stx500. Tried a {garbage-can tuning box}(with it, it boosts to 38). changed wasegate hose. Had Cummins out and they say fuel pressures are good and no codes.

Only way to get it to boost smooth is to clamp off the hose between wastegate and wastegate controller but it still doesn’t seem to pull as good as it should. We have 3 other similar tractors 535 quadtracs s/n79418748 cpl 3087 and none of them act the same as this one. Pull much harder and smoother power. They don’t use a wasegate controller though they have cummins turbos whereas the 480 uses a holset turbo

Looking for any ideas what else to try on this thing. Or any insite to how the wastegate control valve actually functions or why it would make the turbo and or fueling smoother with the hose crimped. Also wondering if I could put the 535 tune and turbo on and just ditch the wastegate control valve altogether