My question in post #20 has not been answered by the OP. So do we have a 100.000 km (mls?, in one year? second hand?) engine really without soot build up, or a wrong diagnostic?
This is not model/engine specific. Diesel engines with EGR recirculate particulate matter (soot) back to the intake. As some have indicated, that is based on a steady state map. If you accelerate above a certain minimum rate (which is really low), EGR is cut off. Also EGR is cut off when outside the emissions box (eg where they measure emissions, or on a EPA cycle where it has little impact on the total). On my 320d that steady state limit is at 2390 rpm in 6th, or about 85kph indicated. Above that 85 kph level road there is no more EGR requested. So that has been my cruising speed. On a 330d/335d, I'd expect that rpm to be higher as you'll have less load on the engine. I do 90% highway. And even with that my intake system is not very clean. I've an ODB reader showing EGR position.
The EGR recirculates dry soot. It comes out of the engine, pre turbo, at 500C or so. That has to be bone dry. So it leaves a dry, powdery coating. The problem starts on the back side of the throttle valve, where this dry stuff mixes with intake air containing oil from the breather.
Oil Catch Cans...certainly a move in the right direction but I have my doubts. Search for "coalescing filters". A bit of wire mesh in a coffee cup sized tin is miles away from where the heavy duty truck industry is moving. And there is flow restriction, too much restriction and these won't flow. Great from no oil carry over perspective. Not so great from a oil perspective, where the water vapours etc can't escape.
I'm running a coalescing filter given to me, stuck in a housing made from diy shop bits.The filter element is I think 4" dia, 8" long. Last time i took it apart it was black on the outside, oil on the bottom, and as new on the inside after at least 20k mls.
And here's the info from Pierburg, you might recognize the components under the hood, as Pierburg supplies BMW and many others. It shows the basic chart where EGR is in operation.