My situation may not have been due to the fuel pump, but the mechanic's
opinion that the FP had an issue and was generating too much heat and thereby
cooking the fuel seemed to make sense. We drove about 30 min to his shop, first
thing in the morning and he said the fuel was much hotter than normal when he
disconnected the lines. We had no problems after the new FP was installed, but
we also broke up our trip home over several days, so not quite as much non-stop
If this can happen with a good FP and working AC, then it's surprising we don't hear about more issues like this in Texas and other hot states.
I was surprised also, which is why my original roadside diagnosis was fuel pump. But thinking on it, the fuel pump doesn't care whether the engine is running or not: It is going to pump X amount of fuel (2.5L/min in this case) to the engine no matter what, and whatever the engine doesn't use gets sent back via the cooler. So engine running or not makes no difference-- except for the fuel cooler.
Now that said, I cannot say the fuel cooler was doing an outstanding job... I didn't have my IR gun with me, but the hand-on-tank-o-meter said "pretty darn warm", could have been 120-130. The AC was running well, I'm just not sure there is enough heat-exchange capacity. Has anyone had one apart?
So for future reference and for those to whom this might happen in the future, the fuel in the tank will cool itself given the chance, by opening the fuel filler cap after you put out your cigar. There's pressure built up in the tank (the fuel is 'boiling'...), so do this slowly and hang on to the cap. I had one launch across the desert the first time I did it, and had to go find it. The fuel in the tank will cool as some boils off at the reduced pressure. You'll know that it's boiling in there by the burbling noises from the tank. Wait until all that's all done and for a little while longer, then put the cap back on and restart the car. The fuel will have cooled to somewhat less than ambient thanks to the rapid evaporation, to somewhere near its "dew point" if you will. From there the pumps will see liquid again, and you can proceed on until the fuel heats up again. If the tank level is pretty low, adding fresh cool fuel will help, not just from the cooling but from the added liquid head available to the pump.
Reminder that the vapor from boiling fuel is a HUGE fire hazard as it ejects from the tank and mixes with air. No sparks or other ignition sources please. Of course you shouldn't inhale mr president. Nice if there's a breeze to carry the vapors away.
Dr Bob, '89 S4 Auto, black.