Perhaps I'm misunderstanding - are you saying that the formation of H2O in the combustion of hydrocarbons is not exothermic?
Yes it is. And to produce the Hydrogen requires an equally powerful endothermic reaction. Plus the energy required to compress, store and transport it before you use it in your car. Hydrogen does not exist in any significant quantities except where combined with other atoms such as oxygen and carbon. It must first be separated from them and that takes energy.
Here's why I wrote that ...
I forget the details ... (thermo WAS 30 years ago) but in the following equation, representing the combustion of propane, CH3-CH2-CH3 + 5O2 --> 3CO2 + 4H20 + heat; **IF** I recall correctly, the preponderance of heat comes from the oxidation of the carbon atoms, because you are oxidizing the 3 carbon atoms, but reducing the 3 oxygen molecules (when making the water molecules /i.e., not oxidizing a hydrogen molecule).
As any good FReeper, I am open to correction. It's been a long time since I figgered out calories ...