System Too Lean/Valve Cover Gasket Leak/Misfire in 3rd and 4rth?

[julianforte]

Hi all. Thanks in advance for your help!

First I'll start with my car/mods:

02 WRX Bug-eye 104K miles
Blow off Valve (BOV)
ECU Flash that's a little less conservative/faster than stock - performed by a friend (has been running perfect with this map for over 40,000 miles until now)

Now for my problems:
*CEL code P0171 with possible causes including: faulty MAF, Large vacuum leak, Lowe Fuel Pressure, or Plugged injector or cylinder misfire
*Tons of build up on passenger side Valve Cover Gasket - build up inside coilpack area in sparkplug nearest passenger (the one w the black connector..)
*I'm experiencing a slight loss of power or "bogging" in 3rd/4th when the turbo is boosting at max. I can conservatively drive in 3rd of 4rth without experience the bogs or misfires; it happens when I get down on the gas and the turbo is really kicking in.


Today I changed the spark plugs because that was the easiest and most logical item on the CEL possible causes list, however, my test drive afterwards showed no difference in the "bogging/misfires". The CEL is gone for now but I'm sure that it will come back since the problem is still there.. I would also like to note that none of the other sparkplug recesses were dirty except for the one mentioned above in the back of the engine nearest the passenger side..

I have reason to believe that my problem is most likely related to a Plugged injector or cylinder misfire since that sounds like what I'm experiencing but I may be coming to the table w an incorrect bias.

I'm ultimately interested in some advice as to what my next step should be.

Thanks,

-Julian

[Donkey]

IF YOU HAVE A MISFIRE CODE ,PLEASE READ THIS!!!