I have Subaru Impreza 2.5i (2.5 NA). My car's engine sputters a bit when turning it off. Specifically it seems like the cars engine keeps turning for a little longer right after I remove the keys.
There don't seem to be any issues starting the car or driving it around.
There are no CELs
The only thing I can think of that's a little strange in the car's recent history is that I put in premium high octane fuel (the gas station was out of regular). I can't definitively connect the two incidents though. In between doing that and now, I also had to boost the car once since it was really cold. I don't know if the ECU reset with a premium fuel type would do anything... again, can't really connect the two. Its been long enough though that I think everything should be calibrated anyway.
The engine has < 40,000 km on it and the car is completely stock.
What are some reasons the engine could be sputtering? What could I try to diagnose the problem?