Sad to hear that they haven't found an undeniable cause for the several turbo failures...
I'm no expert in TBI engines but I've heard a lot about the wrong oil specification delivered by AR on early new TBIs and on scheduled maintenance on AR dealers, that caused a wave of turbo failures on Giulieta QVs...
Something similar to the well known situation of steering oil (GI/E vs GI/R), that cause a lot of steering issues on early 159s.
I've read that AR has already changed oil specs for the TBIs, so this could be a non issue for your case but, just my 2 cents...
Yep - the change in oil specification was discussed at length in a thread started by myself about 2 years back : http://www.alfaowner.com/Forum/engin...e-1750tbi.html
cant remember if we brought this up in the older thread a year or so ago but what or how does this engine releave boost pressure when full throttle then lift off and butterfly shuts ?
what sort of layout is the blow off valve ? mechanical ? electronic valve etc ?
has this been checked out, im sure ive seen mentions of compressor stall breaking shafts on the net(not alfa or tbi related).
heres a description off garret site
Q. What is compressor surge?
A. The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side. Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself. Common conditions that result in compressor surge on turbocharger gasoline engines are:
-A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body
-The outlet plumbing for the bypass valve is too small or restrictive
-The turbo is too big for the application
Q. What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary?
A. A Blow Off Valve (BOV) is a valve that is mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge. Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.
Q. What is causing my turbo to sound like a sewing machine's whistle?
A. The "sewing machine whistle" is a distinct cyclic noise cause by unstable compressor operating conditions known as compressor surge. This aerodynamic instability is the most noticeable during a rapid lift of the throttle, following operation at full boost.
That makes interesting, albeit disturbing, reading.
That 'whistle' described in the last section is all too familiar,
Prior to the original failure back in 2013, the car suffered from an intermittent loss of boost (you'd be on full throttle in mid-overtake and at around 3500rpm when it would suddenly drop the boost to 0) which was blamed at the time on a wiring issue at the lambda sensor. This sudden change in boost pressure probably had something to do with the subsequent turbo failure although at the time the finger of blame was pointed at the oil specification (see my reply to filomix above). Since then, although it's never again lost boost completely, it has suffered on occasions form a hesitancy under throttle with the boost gauge fluctuating by about 0.3-0.4 bar and only achieving a maximum reading of around 0.9 bar - if you lifted off momentarily then reapplied the throttle, it would then deliver the full 1.3-1.4 bar and not happen again for quite a time,
Something else to bounce off the dealer methinks,