when i turn off my car it keeps sputtering

Otateral,
Read my post and others above. The more you can lower the idle speed, and thus close the resting throttle position more, the better it stops dieseling by choking off the air. The more advanced you set your spark timing, the better your mileage and performance, until you reach the knock limit, which depends on many things - gas used, air temperature, etc. Modern cars use a knock sensor and the spark controller keeps advancing timing until it knocks.