850 T5 300Hp - Loss Of Power At High Boost (Rumbling Hesitation)

[Fudge_Brownie]

as the boost isn't that high - doesn't even leave the white of the stock turbo gauge - and I run 95 ocaten fuel

I'm quite certain it doesn't give me 300BHP, because I can't floor it. I'm running a 15G - the ECU is tuned for a 16T. It's the drop-in RICA tune.

So it is significantly under target boost? And no Check Engine light set for misfires? Sounds like a large boost leak you haven't found yet. A common one people miss is at the throttle body. The smaller hose running to the IAC develops a crack at the base which will only reveal itself when pressurized.

[lookforjoe]

If all the ignition components are known good, and although used came from a known good running car, I would agree on checking the IAC/TB hose - it can rupture underneath, and will suck in at idle, and only leak over around 12psi (that was my experience) car will if course go flat at that point.