[Diesel_Dave]

I don't think there would be much of an effect. Like oilpan said, I don't thing any "pre-warming" is going to affect the temperature at injection hardly at all. I guess slightly hotter fuel might possibly help FE by decreasing the ignition delay--but my guess is that effect would be almost nothing. This is especially true to common rail systems, where the high pressure pump will have pressurized the fuel up to 10-25,000 psi. Ignition delays are very, very short with common rail systems anyway.

On an older mechanical-style fuel injection system, I'm not sure what the effect would be. Changing the fuel temp will change both the fuel density and bulk modulus. Both these factors determine how quickly the fuel pulses travel down the fuel lines, thus it could affect the injection timing. Of course if you wanted to achieve that effect you could just change the injection timing without warming the fuel.