If your engine is running on for 10 seconds and is smooth running not deiseling you may have a different problem if you added an electric fan. Did you recently add an electric fan?, if not and the engine is dieseling then ignore this post.
If you did add an electric fan it can produce enough power when you turn off the engine while the fan is slowing down to keep the motor going if you direct wired it to your 12 volt system. This problem can be aleviated by installing a diode inline on the hot side or even better by installing a relay on the fan.