I have a USDM 02 WRX pretty much stock except a COBB ram air intake that is hesitating and throwing a misfire code every few weeks. I've had the car since 4/2001 and never had any problems with the thing until a few months ago when my check engine light came on. I bought a tactrix cable and downloaded ECUExplorer, code it's throwing is misfire on cylinder 2. Cleared the code and reset my flash and immediately noticed that my car was hesitating during acceleration now. Throws same code after a few days, clear code and reset flash, and repeat. I've changed out my plugs (which were NGK Iridiums with maybe 30K miles) to Denso Iridiums, no change. Changed my fuel filter, no change. I've run Redline fuel injector cleaner several times now and it SEEMS to have gotten better, but that may just be subconscious hope. What's weird is that the hesitation only occurs around 4000 RPM's and in 1st gear is hardly noticeable, and gets more noticeable until when in 4th gear the car does not want to accelerate past 4000 RPM's.
Forgot, I did replace the intercooler air intake gaskets with new ones since a few years ago I replaced my starter and didn't replace those gaskets or use a torque wrench at that time. Also, when replacing my starter and re-installing the BOV, I over torqued one of the bolts and it completely tore out the threads so that the BOV could be a little loose. But if this were the problem, why would it start causing problems now. I put teflon tape around the bolt and was able to get some descent torque on the bolt. I also replaced BOV gasket as well.
Sorry for the long post. Any suggestions?