[HUMTECH]

How many miles on the truck? What year? Sounds to me like the intake gaskets are leaking. The 6.0l uses a dry intake(no coolant flow through) and when the gaskets start to leak it is a vacuum leak. Un-metered air into the combustion chamber causes the truck to run lean. Best way to check it is have a scan tool hooked up and run the engine at a cold start up and monitor the fuel trim short term percentages and spray intake(carb) cleaner where the intake and cylinder heads meet and if the percentages change abrubtly your intake gaskets are leaking. When it gets real bad you can notice a rough idle and even miss fires when cold starting.