right in thinking that once this arm is disconnected we shouldn't have any trouble with the flaps
The motor controls the “active” swirl flap. All of the others are “slaves” and they are activated by a bar connecting the active flap with the slaves (this is the bar on top of the inlet that pops off).
In my case, the plastic casings on top of the inlet manifold had started leaking and I was leaking boost.
In other peoples’ cases, the swirls stop moving (because they are gunked up) and this means lack of flow.
In other peoples’ cases, the swirls get gunked up to the degree that the plastic flap snaps off and makes its way into the engine. Not a big issue if you have a metal manifold and plastic flaps. Imagine having a plastic manifold with metal flaps, as is fitted to some of the later 1.9JTD engines (including the 159 – your engine will ingest the metal and basically break itself!
In order to not have any swirl issues, they will have to be removed! I have enquired about this and, to have the swirls removed and the holes welded, it will cost around £50.
Another option is to have a fabricators machine some blanks and have them fitted. There is a kit available on ebay but I would prefer to have something bespoke made and fitted “properly”.
However, in the end I just had a new "stock" manifold fitted. But I do have the original manifold and might have that played with...you know, for next time!!!
Hope that helps