Usually, from experience and what I have gathered, loss of power appears to be the main sympton with decreased mpg as a secondary
It seems the valve normally sticks in the open position, so the turbo doesn't get quite as much exhaust flow as it should get, meaning that turbo lag feels worse. It usually feels more like an old-school turbo diesel - nothing, nothing, nothing, whoosh, rather than coming on boost more gently.
Fuel economy suffers too, as due to excess exhaust gas displacing oxygen in the cylinders, part of the fuel injected isn't burnt. This reduces the cars economy.
There is a lot of in-depth discussion of this over on AlfaOwner.com, this engine is also in the 147, 156, GT & new 159. A permanent fix is to clean out the EGR valve, so it is working freely, and then to cap it off at both ends. The ECU needs the EGR valve itself to be operating correctly or it throws up a code, but it cannot tell if no exhaust gasses are flowing past the valve. Car will still pass MOT emissions test although will be approx 10% more harmful emissions.
There are also "secondary swirl valves" in the inlet manifold, these close partially at low rpms and low load, to narrow the aperture for air entering into the cylinders - this increases its velocity, aiding volumetric efficiency. The same thing can happen with these valves, they can gunk up and jam up and cause the management to throw a hissy fit. This is happening in quite a few high mileage Vectras, which also have the same 1.9 16v diesel engine fitted, plus one or two Alfas.
Basically, what causes it is the soot from unburnt fuel mixing with oil mist that weeps through the turbo seals, back into the inlet tract. The oil weepage is normal, but long term it causes a sticky black tar-like substance that gunks up the inlet manifold. The amount that I scraped out of the inlet manifold of my old Alfa 2.4 JTD after 110k miles was shocking, you could have half-filled a Coke can with it :yuck:
Regular cleaning of the inlet manifold would hopefully prevent this from happening, but capping off the EGR pipe has to be the ideal solution IMO.
A good treatment of EcoTek Powerboost used to work wonders for my JTD, its like a fuel system cleaner but for the air intake, you spray it into the inlet manifold while the engine is running and it dissolves gunk and burns it. You can tell on the first run afterwards how much cack its cleared as the amount of smoke the car makes is shocking!
Hope my waffling on has helped some of you
