D13 White smoke, 6th injector mechanical fault.

Hello good people. So I got a 6th injector mechanical fault code on a Volvo D13. It started about 2 months ago but the fault would become "inactive" after truck ran for about 5 to 10 minutes. The exhaust pipe would spew white smoke for the first 5 minutes after starting the truck but then it would clear and everything seemed normal, except I noticed some rough idling. Truck ran fine at high RPM and pulled strong. No apparent loss of oil or coolant. About a month ago the white smoke coming out of pipe became constant (it will not clear after running engine for a while as it did before). The smoke will clear however as soon as engine will rev up past 1500 RPM. Went ahead an changed the injector (tested the old one and they said it was bad). Of course, the problem remains the same. Still having white smoke coming out at low RPM, rough idling, hesitation when in low RPM and same fault. The only improvement is the engine now starts OK, whereas before it would have problems starting sometimes. The mechanic says it has got to be the cup then, but when changing the injector I asked several times if the cup should be also changed and he said no. NOW he says yes.
I need some guidance because I don't wan to keep throwing money away if the problem may be somewhere else. Thank you all for any help you can provide!

 

The truck does not have DPF. Still even if it did it shouldn't be throwing the injector 6 mechanical fault code even if the smoke were a DPF issue. I'm starting to thing that I got unlucky and got a bad replacement injector....or it is the injector cup as the mechanic says but the symptoms do not seem to point to a bad cup...

 

The entire engine wire harness along with the camshaft was changed a little more than a year ago.