Rough Idle and check engine light at 105k solved. Our Scion Ia's are direct injection engines. Direct injection engines have a problem with Carbon fouling and if you have a rough idle please have the Carbon cleaned from your intake tract and valves. This should cost about $300. Mine went all the way to a P0300 misfire code and it costs me $600 to remove the intake. Dealer tried to sell me new car and said engine was no good. I reputable shop call Salem Brothers (AZ) was willing to try and fix the Carbon build up and the engine is fine. Most dealers will just say you need new engine and try and sell you a new car. There are risks with having let the carbon build up for a 100k. It may not be possible to clean all the carbon out and dealers won't take the chance. Shop said to clean Carbon every 20k. I tried everything I could think of to fix my rough idle. I had a warranty to 100k and dealer's just told me that there was nothing wrong. Manuel says nothing about this needed maintenance. This affects other cars with direct injection as well. Rough idle is more noticeable with A/C off.
Seems rough idle at 500rpm with A/C off is motor mounts. Violent miss fire is carbon build up in the intake valves.