[snapdragon]

Well, the gunk is usually oil mist from the turbo that has combined with smoke/soot from the EGR.

If your EGR is disabled, it shouldn't really continue to build up, but the symptoms shound like EGR stuck open.

When the engine requests EGR, it looks for a coresponding drop in air metered by the airflow meter (that's how it meters the EGR). So if the EGR is stuck open, it doesn't see any drop and keeps increasing the duty cycle until the exhaust smoke is just black smoke.

I remember you changed your engine for a later one, so not sure if you have the vacuum operated EGR of A8 engines or the the later electronic one still used on current models. Assuming vacuum - if you pull off the EGR vacuum pipe with the engine running, can you hear it snap shut?