I would buy a can of electrical contact cleaner (made by CRC, $5 at Autozone). Sometimes dust and fine dirt particles can collect onto the mass airflow (MAF) filament inside your MAF housing. All you have to do is disconnect your MAF housing from your air filter housing and spray a little bit (it doesn't take much!) of the cleaner onto the filament (look for the little metal wire inside the MAF housing). Even a little bit of dust can skew the voltage readings that get transmitted to your ECU, which causes your fuel trim to run more rich. It's a quick and easy fix, so it's worth a shot!
