It doesn't work, the plans they give you won't crack enough water to run an ants motorbike let alone a car. All you end up with is, is a crappy water electrolysis machine.
I do remember them building and testing this thing on Mythbusters and proved it was bogus, though weirdly they briefly showed you could run the engine on hydrogen gas, even though it tended to explode as the engine isn't set up to handle it. So if you could make a powerful enough hydrogen generator you could run your car on it with modifications, though such a machine wouldn't be efficient to be viable.
I'll link the PDF i have which is the same as the water4gas, or the other ones out there.
How to convert your car to run on water (pdf)