This is a common problem for all modern diesels, it isn't restricted to Fiat or the Multijet engine.
Exhaust gas is thrown back into the engine via the EGR when fuel injection in nil/minimum and the engine is likely to be running lean, say on over run or a steady cruise, the exhaust gas replaces the available oxygen and reduces the production of nitrogen oxide (NOx).
NOx is a green house gas and it's production is higher when combustion temps are very high (temps are higher when the engine has less fuel injected, hence lean), so throwing exhaust gas back in at this point reduces the temps as it has little oxygen in it.
The system works, but it does have a small overall cost, there is a slight reduction in economy, around 2 or 3%, so they say, but NOx production is vastly reduced.
Unfortunately, the exhaust has lots of soot in it, this gets dragged through the combustion chambers along with engine oil/vapour from the crankcase breather that is plumbed into tha air intake, as these days they aren't allowed to vent the crankcase to atmosphere, so it all gets pumped back into the engine to be burnt.
(or would if it was actually injecting fuel and firing, but at this point it isn't!)
As diesels run much higher compression, the pressure of the oil under the pistons tends to fluctuate much more than a petrol and needs more venting, so they can throw quite a lot of oily vapour out of the breather.
This oil/soot makes a lovely cocktail of greasy gunk in the intake side of the engine and the engine can make quite a lot of it.
It starts gumming everything up as it cycles around and around, the two most common failures due to this are the EGR it's self jamming up and the wastegate/variable vanes sticking as the gunk makes it's way through without being burnt.
Earlier diesels (methinks euro 4 and before) didn't have to meet such tight emission regs, so it was easy to get away with blanking the exhaust feed to the EGR without the car's PCM complaining as it wasn't being monitored by the PCM.
Euro 5 and onwards regs required the PCM to monitor EGR operation and flag a fault if it failed (bugger), so restricting it's flow would become a problem and the PCM would flag a DTC.
This is very common on the Ford Duratorq (Mondeo/Jag X Type) which production spanned quite a time frame.
Early EGR's (late 04) could be fully blanked without trouble, later cars couldn't (some got away with reducing the flow via a blank with a 8mm hole).
So with the Multijet, at best you could restrict the exhaust flow to the EGR by fitting a blank and experimenting with different hole sizes through it until the MIL stayed off.
Or disable it through reprogramming the PCM.
I've seen this done on a few twin turbo Jag V6's with two EGR's, one on each bank, though the EGR's were disabled partly open (10%) to compensate for any excessive combustion temp problems that might occur when injection stops (over run).
Another option would be to tackle the other cause of the gunk, the crankcase breather/oil vapour and try and to stop/restrict the oil vapour entering the air intake.
You can't vent to atmosphere and drip oil all over the road for some biker to skid on, so some way of condensing and catching it is needed.
I tested this on my old X Type, I cleaned the EGR but left it unblanked, then sent the crankcase breather through a catch tank designed for diesels
http://www.allardaluminiumproducts....lf-seat-skoda-audi-tdi-vag-group-engines.html
(not a cheap ebay one as it needs enough bigger ports to allow enough flow through it and is baffled to trap the oil, but it's fairly bulky)
and back into the air intake.
12,000 miles later the EGR was still clean as a whistle as the dryish soot couldn't stick to anything and no problems with turbo vanes, egr or smoking.
