Thanks for the explanation RS! I have heard about cylinder 4 running lean on the stock system. I've also heard that a manual boost controller magnifies this problem - is this due to the same reason (hotter fuel, not compensating for boost)?

Just for the people out there, you can get a Walbro online (if you're game) for around $160 and that's all you need to do the install. The Bosch can be acquired for $260 and how much more are the required fittings? And is the Bosch really that loud? (my Walbro is pretty much silent - have heard it prime a couple of times when I've had the window down in the garage at start up).

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MY99 WRX (Sold)
MY06 WRX 241hp 430nm atw (Sold)
MY09 BMW 135i 247kw atw (Current)