Even if you have stool left autolysis will eat it up and use it for food.
My suggestion is to first gain control of your habits and then do a Water Fast properly.
The only and I mean only way a Water Fast will be successful is if and only if you change your diet to reflect plant based afterwards. If you choose to go back to how things were then you will be fat and fatter in no time. Nothing will prevent this is ... because:
1) A waterfast gets your metabolism efficient which means you will require a lot less food afterwards.
2) Your body will not need as much fuel after a Water Fast so the person has to learn how to eat only to hunger.
Without 1 & 2 you will put all the weight on and then some.