A few things to consider and check!
Does it still have EGR system in service? If so, it is very hard on engine parts including turbo. All that soot going through the intake, combustion chamber and turbo, mixing with oil vapors makes one heck of a mess and gums everything up. Makes soot pies in the intake.
If you still have an EGR valve, it can stick open and you will lose boost pressure backwards through it allowing boost pressure to bypass engine and dump into the exhaust.
If you still have a FCV in service, they are notorious for the mechanical gears in the operator wearing out and sticking the valve partly shut or almost fully shut preventing air flow and boost pressure from entering the intake manifold. When this happens, it will severally engine and turbo performance making it act like it is in limp mode or it has a governor on the engine.
Check all your vacuum lines and hoses very carefully going to the turbo controls, vacuum solenoid, and vacuum modulator valve, and especially the one from the vacuum modulator over to the turbo actuator. The factory vacuum lines can get holes in them or the plastic ones can get briddle and snap or get to hot and collapse preventing proper turbo vane control.
(See second picture below of bad factory vacuum lines/hoses) Replacing all the factory vacuum lines and hoses with silicone ones is a good mod as the silicone ones are impervious to heat and oil and won't collapse.
You can also do a vacuum solenoid bypass test to make sure the turbo vacuum solenoid is not causing problems. See pictures below.
Improper boost will really make these engines run very poorly and produce excessive black smoke.
If you still have a CAT in the exhaust system, it can stop up and severally limit turbo and engine performance.
Also, check the inlet side of your turbo and make sure the rubber pipe between the air cleaner box and turbo inlet has not come apart and allowed a piece of rubber to get sucked into the turbo.
Having a mechanical boost gauge can tell you if you having boost issues.
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Supporting Vendor and Moderator of LOST05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount98 Dodge Cummins 24 Valve