92 7.3 and it is turbo'd. It has 100k miles and not sure on hours but ALOT im willing to bet as its in an ambulance.
symptoms- Starts fine when cold. once warmed up and hot i can shut the truck off let it sit for 5 minutes. Turn the key and the engine turns just a few revolutions. If let cool down it turns over with no problem. If a battery charger is hooked to it and hot started it almost sounds like a rod knock(some sort of knock) and then it will fire.
Its not the starter, its not the batteries. I have new batteries and a high torque mini starter on there. Getting 12.5 volts across the starter. what kind of engine problems could cause this? hydro lock? i wouldn't think vapor lock. Timing?

 

Try this simple test: When hot and will not start, pour cool water (NOT ICE COLD) over the injector pump. If it cranks up, then you have what is called HEAT SOAK. This is when the tolerance of the injector pump has worn pass it's limits internally and the pump needs to be rebuilt, cooling the exterior of the pump constricts the metal and allows it to crank. I'm pretty sure this will be the problem since ambulances have a high idle time, compared to mileage.