You've got a high pressure oil leak. What year truck? Could be crankcase to head tubes, Injector seals, HPOP quick connect fitting, or HPOP itself. Once the oil gets hot and thinner it loses prime and won't build pressure once you turn it off. You can put shop air into the ICP sensor port while you energize the IPR valve and check for air leaks to pinpoint the area with the leak. Smaller leaks still may be hard to find without looking.
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