My over boost troubles started just before Christmas.
On a motorway trip in heavy rain.
During the trip we wend several times true some very bad spray, coming from trucks we have over taken.
That sort of spray where you think you sit in a car wash and you are almost blinded by all the water.
All the usual symptoms of over boost;
Once you get near 2500 rpm, the engine management light comes on and all power disappears.
Overtaking anything becomes a no no.
I finally traced it down to the (N75) VNT solenoid.
(The solenoid sitting behind the slam panel on the 2.4 JTDís.)
Mine had socked up water, and would not operate anymore.
The permanent vacuum causing the over boost once the cut rpm where reached.
I got a new solenoid from Alfa
(over £100, if someone knows where to get them cheaper let me know for the next time)
and all is fine since.
Now, it appears that we are not the only ones suffering that problem.
Fiat had a recall to some diesel models where the VNT solenoid was moved from the wheel arch
to a location near the battery to avoid water getting into the solenoid.
I intend to move my solenoid away from the slam panel and locate it near my battery.
The cost should be minimal, with some 5-6 foot of vacuum house and some cable ties perhaps.
This little modification could save some trouble next time, when we get another torrential rain on the motorway.