[ShadeTreeMech]

Vapor lock was a common occurance on older carburated engines, and the cause was overheated fuel in the lines. The net result was not being able to start the engine.

I would think warmer fuel would be fractional thinner, and therefore would come out of the injectors faster (maybe uncontrollably?) and best case scenario might possibly net worse economy. Worst case you have a volatile combustable chemical that now has more energy instilled in it already and could possibly explode easier; good thing if it happens in the cylinder, bad elsewhere.

The fuel that goes into the fuel rail generally also returns to the tank, so the fuel in the tank would warm up, which would encourage evaporation. How will the evaporative canister handle the extra vapor? I don't know. Can you loop the fuel back into the rail? sure you can, just crimp the return line; it works in a pinch (BAD PUN ALERT!) if the pressure regulator goes out. But this would likely cause worse fuel mileage unless the injectors were configured for higher pressure (I assume...) i would assume the higher pressure would strain the fuel pump, possibly causing premature failure. Electric motors cope with additional strain by drawing more amps which causes things to heat up. A hot fuel pump isn't necessarily dangerous, but it will eventually fail. Considering a normal fuel injected engine has the injectors millimeters away from a VERY hot arena, I suspect those gas molecules are vaporized very well by the time the spark goes off. So it may be a moot point.

as mwebb pointed out, heating the air will have a bigger effect, especially considering the amount of it being used. And I've yet to hear of an explosion from overheated air. And it'd be quite easy to rig up a HAI compared to heating up gas under pressure, and likely a bit cheaper. I'm actually toying with the idea of pulling in intake air from the nether regions of the exhaust pipe to warm things up a bit. A bit of exhaust is bound to lower my pumping losses and raise the temp of incoming air.

I was just rereading the post and realized an important point. The assumption behind wanting to heat the fuel is to hopefully extract more energy from the fuel. I wonder then, what percentage of the fuel is combusted in a well maintained engine? Seems I haven't smelled a modern car running "rich" unless the o2 sensor went out or some other important component was faulting, so it makes one wonder if heating the fuel will extract anymore energy out without a redesign of the engine itself.