Run Your Car On Water?
The point is that you cannot get more energy out of Hydrogen and Oxygen made through electrolysis than you originally put in. Therefore, trying to run your car off of Hydrogen you create using energy from the engine is the same trying to make a perpetual motion machine. Like plugging a generator into a motor, that powers the generator. Efficiencies kill you and the machine doesn't work.
If you search for this subject, you will see that this topic has been covered many times already.