flip213

M_sprank I did not take Cobbs advice because if my problem truly was a compression issue then i would losing power and the engine would be also idling erratically, i've seen compression problems before and mine definitely wasn't it. Also it would be a progressive decay. My stumble happened from one day to the next. So to let everyone know on this forum i have found my problem, a while ago, but have not had the time to respond to this thread. It was my fuel pump not delivering the correct fuel pressure (tired pump). Check the fuel pressure you are getting from your fuel pump, the manual states you should be getting 41-46 psi from the fuel delivery hose after the regulator at idle with the vacuum hose disconnected and 33-38psi with vacuum hose connected. I was 6-7psi below the spec. Bought a new deatschwerks D65C pump and stutter is gone and i've noticed stronger and much smoother acceleration.

TYPE OF STUTTER/STUMBLE Steady throttle/Increasing throttle/Both: Rapid increasing throttle, WOT MODS Intake: K&N Panel filter Exhaust: STI up, LP Down with O2 out of the flow and high flow cat, stock after downpipe Fuel: Stock fueling, 91 octane Turbo: Stock Engine Management: Cobb OTS Stage 2 91 v300 Tuner and revision of tune(if you don't mind): COBB AP v2 MAINTENANCE Sensors changed: none, MAF is clean Plugs changed: NGK Iridium IX with approx. 40000km on them Coil Packs changed: yes last week OTHER SYMPTOMS Check Engine Light: none Etc: Car stumbles/shutters under WOT under 3000-3500rpm with no backfire or black smoke. not sure if i'm hearing popping out the intake. no stumble on light throttle or steady throttle. I've done a datalog and sent it to COBB with a response saying i should do a compression leak check. Datalog also shows knock indication and reduces boost because of knock. Knock normally occurs just before peak boost which reduces boost. Boost target is 16.2psi once under full load and passed rpm range of the stumble, the boost will stabilize and peak at 15.7psi. I don't believe its boost leak. Pulled spark plugs and they all indicate good burn. one had slightly more carbon deposit but acceptable. No really white (lean) no really black (rich) plugs, all nice light brown. The knock isn't caused by lean mixture (plugs would be on the whiter side) I've had the same setup for 3yrs without a problem RESULTS What has and hasn't worked for you:Nothing If something worked, how long has it been working for: Anyone have any ideas? How long do the iridium IX last for generally? cam sensor or knock sensor possibility on the way out? OCV?