You need to drain the coolant until the radiator coolant level is below the
Thermostats. For the 5.7L only, remove the following items: the bolts or studs, the coolant outlet, the thermostat from the housing, and the gasket. For the 7.4L only, remove the following items: the bolts or studs, the coolant outlet, the thermostat from the housing, and the gasket. Then you can clean the thermostat housing and the cooling outlet sealing surfaces.