Accent diesel will not rev above 2500 rpm, very slow to rev to that. no air leaks, no air restriction, changed air flow meter, injector leak back test ok, exhaust not blocked, throttle position sensor reading ok, checked t-belt all ok, no fault codes recorded on engine ecu ?

I had a case tractor with a 4 cylinder diesel, dual heads, in-line cylinders. It had a similar symptom. Turned out to be a cracked head. Only one of the two. Water pump had previously failed, this led to overheating and cracking of the head.

It could be as simple as a vacuum leak. The emissions are probably changing (lean or rich) at higher RPMs. You need to monitor the emissions & go from there. Have a qualified tech ck it. The high RPM issue will require some professional equipment.

not necessarily
when diesels knock and diesel at low rpms it generally indicates a dribbling or leaking injector/s
have an accredited diesel repair shop check it out
If it has some miles on it and the injectors have never been done then the injectors are a fair bet

Have the fault codes read Replace the faulty units with genuine one because most aftermarket units are not set
for OEM cpu units and are not completely compatible

On many diesels, when I read codes they mention an egr valve, but what they are really talking about is the throttle actuator. What I don't understand is why you no longer have any codes. there are some codes which do not turn on the light on the dash because the computer hasn't decided yet that it is a hard code, so check for them anyway. The symptoms you are describing usually turn out to be either the throttle actuator, fuel rail pressure regulator, or a ripped turbo hose

u have a leak u need to check areas around injectors

its drawing too much air in to the intake bypassing the injectors/air flow sensor check the inlet manifold gasket and all the seals like injector seals or gaskets etc

Try running some injector cleaner through it , also check to see if there are any air leaks between the throttlebody and the flow meter ( easy way to test is to spray CRC or WD40 at the air pipes while the car is idling, if it revs up while spraying a particular spot, that is where your air leak is.) An air leak between these two parts will let air bypass the flow meter and the engine won't increase the ammount of fuel going to the injectors as you accelerate causing the engine to lean out and hesitate or stall.

bad mass air flow meter replace it also check for a ecm update at the dealership goes by vin pretty common problem

Change all the filters (fuel and air). check and see if all your spark plugs are firing or just change them. if it is fuel injected make sure your injectors are working. Hope something there helps get it running good.