Here are directions to replace the thermostat:
5.0L, 5.8L and 7.5L Engine
See Figures 2, 3, 4, 5, 6, 7 and 8
Fig. 2: Unbolt the thermostat housing
Fig. 3: Pull away the thermostat from the housing
Fig. 4: Scrape the gasket material taking care not to gouge the aluminum mating surfaces
Fig. 5: Thermostat spring always faces down in all engines
Fig. 6: On gasoline engines, turn the thermostat CLOCKWISE to lock it into position on the flats in the outlet elbow
Fig. 7: Thermostat positioning
Fig. 8: Thermostat installation for the V8 engines
The gasoline-powered 5.0L and 5.8L engine thermostat is mounted vertically on the front of engine, the 7.5L engine thermostat is positioned vertically.
Drain the cooling system below the level of the coolant outlet housing. Use the petcock valve at the bottom of the radiator to drain the system. It is not necessary to remove any of the hoses.
Disconnect the bypass hoses at the water pump and intake manifold.
Remove the bypass tube.
Remove the coolant outlet housing retaining bolts, bend the hose and lift the housing with the hose attached to one side.
Remove the thermostat and gasket from the intake manifold and clean both mating surfaces.
For 5.0L and 5.8L engines, coat a new gasket with water resistant sealer and position it on the outlet of the engine. The gasket must be in place before the thermostat is installed. Install the thermostat with the bridge (opposite end of the spring) inside the elbow connection and the thermostat flange positioned in the recess in the manifold.
For the 7.5L engine, install the thermostat in the recess, then coat the gasket with a water resistant sealer and position the gasket on top of the thermostat.
Position the elbow connection onto the mounting surface of the outlet. Tighten the bolts to 18 ft. lbs. (24 Nm) on the 5.0L and 5.8L engines; 28 ft. lbs. (38 Nm) on the 7.5L engine.
Install the bypass tube and hoses.
Fill the radiator and operate the engine until it reaches operating temperature. Check the coolant level and adjust if necessary