Originally Posted by
DMCMW Dave
Something else to think about with respect to the comparison between the fuel pump/line hoses and the injector hoses. In the main fuel lines, the fuel flow is pretty significant, on the order of a gallon every few minutes since the system is recirculating. This holds the temperature of the hoses near the tank at not much over 120 degrees. (I'm sure Dave M has measured it but I don't recall the number!).
OTOH, the fuel in the individual injector lines is moving very slowly, i.e. only at the rate of consumption of each injector. So not so much cooling effect, therefore the temperature of the fuel in those lines, and the lines themselves, approaches the temperature right on top of the engine. This makes the composition of those lines even more important than the lines at the tank.