If you brake in vehicle when it has engine off and it's moving, the engine turns on that it shouldn't do. It should remain off instead. It makes sneaky approach a little bit troublematic.
If manual gear isn't implemented, it could be fixed like this:
When move forward / move backward key is pressed, the direction of movement should be recognized – if the current movement in z-axis is opposite to the key pressed, the engine is not turned on.
If manual gear was implemented, it could be fixed simply so that braking never turns the engine on.