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Discussion Starter #1
2004 WRX, 179k miles, 5 speed, bone stock

Shortly after purchasing from 2nd owner it developed a stutter under throttle from approximately 3800rpm up, if you go feather light on the pedal its smooth and the speed/rpms will go up... eventually. In neutral it revs just fine, this happens only under load. Initially had some CEL’s, misfire 1 and 2 and knock sensor P0327, we changed plugs, coils, knock sensor, MAF sensor, air and fuel filter, didnt fix problem. I have read many posts on different forums, really thought my issue was a bad fuel pump or clogged sock, does not seem to be the case. Brought to my mechanic and here is what he did:

-checked fuel pressure while driving, hits near 50 psi before engine starts cutting out, removed pump assembly from tank and the pickup sock looked perfect and clean.
-pulled off exhaust to check turbo for loose shaft, binding, etc, its perfect
-checked all exhaust plumbing, intercooler pipes, vacuum lines, anything where there might be a leak or clog, everything appears solid
-hooked up his diagnostics scanner and drove it, besides the new knock sensor throwing a P0328 code, everything is coming up normal
-drove with out MAF sensor and then again with air/fuel ratio sensor plugged in, no change

I still think its a fuel issue, he says no based in his tests and gauge reading. Anyone have ideas on what the cause might be?
 

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Discussion Starter #3
Had a chance to head over to mechanics shop and try a few more things. Car starts, idles and runs ok using partial throttle and sometimes pulls ok under light throttle up until 5k or so then you get the bucking nonsense. Anytime you sink your foot into the gas the whole bucking/stuttering nonsense starts. Strange, you can hear noise from under hood when it happens, sounds like air releasing? Once the mechanic gets around to it, hes going to smoke the intake and check for leaks.

Here is what I tried...

-disconnect boost solenoid, no change
-tank was almost empty so put some fresh fuel in, no change
-disconnected vacuum line from BPV and plugged, it got worse, maybe there is a connection?
 

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Had a chance to head over to mechanics shop and try a few more things. Car starts, idles and runs ok using partial throttle and sometimes pulls ok under light throttle up until 5k or so then you get the bucking nonsense. Anytime you sink your foot into the gas the whole bucking/stuttering nonsense starts. Strange, you can hear noise from under hood when it happens, sounds like air releasing? Once the mechanic gets around to it, hes going to smoke the intake and check for leaks.

Here is what I tried...

-disconnect boost solenoid, no change
-tank was almost empty so put some fresh fuel in, no change
-disconnected vacuum line from BPV and plugged, it got worse, maybe there is a connection?
You created a huge leak in the vacuum system and the car acted appropriately
 
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