It sounds like the big maf.

 

One of the mechanics over at the Subaru dealership told me that when you upgrade any part and you don't reset the ECU (basically unplugging the battery), then it may not learn that there's something new there. I also remember reading another post that if you don't reflash your ECU and you have an aftermarket part, then the ECU won't acknowledge it either. I'm not quite sure but these are my "educated" guesses and sorry if I'm wrong.

Remember, I'm not the one saying, it's what one of the "wise" people working at a Subie dealership said.

 

Many sti owners had installed FMIC and reported no problem whatsoever, maybe its the MAF unit. Try reintalling the original set up and see what happen.

 

The Big MAF is your problem.

The Big MAF needs a tune to work properly because it changes the meaning of the numbers it reports. Its a very useful device, but you need a tune to adapt to it.

 

Good link on the big MAF issue.

http://www.pdxtuning.com/technical_tuning_big_MAF.htm

I just realized this thread is kinda old. Did you get the tune? How did things turn out?

 

its VERY dangerous to use a larger maf without tuning since you will instantly run lean