If it is heat sink, it's caused by wear in the injection pump. As your truck runs everything obviously gets hotter, then as it sits for a while with no fuel circulating through the pump to pull at least some of the heat away, it heats up even more. The tolerances in the pump are insanely close and under perfect conditions the thinner hot fuel still goes where it's supposed to. When there's wear, the hot fuel is thin enough to sneak past the areas that are worn and not go out to the injectors with the required pressure or volume to start the engine. If you managed to get it running it would run ok due to higher internal pressures thanks to higher rpm spinning the fuel transfer pump.
Before you condemn the pump check a few other things out. Make sure your lift pump is working like it should, and check the condition of your starter and batteries. If fuel pressure to the injection pump is low, or cranking speed is slow, it can cause a similar issue. The injection pump needs flow and speed to be able to produce the volume/pressure of fuel called for to open the injectors and start the engine.
If you're leaning towards it being heat soak there are a few tricks to buy you some time. The safer option is to mix some 2 cycle oil or filtered waste motor oil into your tank to help thicken the fuel. The other option does involve slowly pouring water over the pump to cool it down. I really see this as more of a last resort because you risk seizing the injection pump if not done just so. That not only trashes your pump, but takes away your core value as well. Im sure someone with experience in these methods can give you more details and hopefully will chime in soon
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