After 60 pages of fleshing out what the boost leak is, what blocking it does, and how to fix it without angering the ECU program parameters, here's the full monty:
*The stock evaporative emissions setup is designed such that the car purges fuel vapors from the charcoal canister in two fashions; 1) most commonly while running the engine within turbo-boost, and 2) while the engine is at lower load in vacuum operation. In order to correctly purge the vapors in both scenarios, a split plumbing is required where one outlet is at the turbo compressor inlet and one is in the intake manifold behind the throttle body. Fiat decided to take a strange design twist on their plumbing to the turbo compressor inlet, and implemented a venturi T fitting and use boost pressure to force vapors through a complicated section of line. This solution creates a boost leak and unnecessary vapor restriction, and is in my opinion a sub-optimal evaporative emission solution on top of unusual versus the rest of the automotive industries' solution.
*I'm going to gloss over the variety of concepts all of us tried to solve the problem of fixing the boost leak while not creating a Check Engine Light (CEL) scenario. CEL's will occur when specific evaporative emission parameters aren't occurring as the Engine Control Unit (ECU) is expecting based on its sensor readings. These aren't necessarily an indication of a performance problem per se, but could also demonstrate the relatively narrow range of programmed behavior the ECU is expecting. It was my goal to find the operational endpoints of the ECU's evaporative emission strategy while also fixing the boost leak to improve drivability performance and pull the turbo compressor map away from the choke line during high power operation.
Dare I quote myself from the Free horsepower? thread:
*The next step is to take out the evaporative emission lines that are causing the problem and install the new lines. Here's a diagram that shows what needs to be happen. Essentially, the line with two "X"s goes underneath the intake manifold and then up to the Venturi T needs to be cut out, with a rubber cap on the leftover T at the left-side "X" mark. The small loop in red is reusing the original 3/8" hose U-shape. The green dot is where you re-use the factory check valve embedded within the "X"'d out line. The long red line is where you plumb into the turbo compressor inlet line.
*Here is a Bill of Materials (BOM) that Shaggie put the link together that shows what needs to be ordered. Keep in mind some of this may be available in single quantities at your local auto parts or hardware store. Don't order a pack of 10 unless you really really love extras!
*Here is the video I put together that hopefully shows what is tough to explain in text:
The result should speak for itself during the 1st test drive. Part throttle actuation has greater resolution and control, and boost builds exactly as your foot commands instead of being erratic and over-reacting. You may also get an extra HP or two, but I haven't verified that, it's just theory. You should not get any CEL's, but this is not a guarantee as I cannot account for individual vehicle tolerances.
Here's a datalog plot I did that shows part throttle pedal versus MAP through casual 2nd and 3rd gear acceleration. Note smooth correlation between foot movement and boost changes. Not erratic at all.
I also have hours of datalogs that show the vapor pressure is similar to stock with this hardware configuration, so I'm confident the evaporative emission system is operating in a safe and predictable fashion very consistent with the OEM plumbing but exchanging the boost leak for low restriction emission line routing.
Cheers and I hope everyone finds this helpful,