What codes were stored? Backfiring can be multiple things...... from lean condition (vacuum leak, maf skewed lean, all the way to stretched/failed timing chain.
That is what my 99 boxster did too. I replaced the MAF, even though there were no codes for it. My guess you have very poor performance at higher rpms too. Take the sensor out and clean it, and if it helps, then get a new sensor.
Hybrid Mode
