1990 Ford F-250 thermostat

The diesel engine thermostat is on the front side of the intake manifold

The factory specified thermostat does not contain an internal bypass. On these engines, an internal bypass is located in the block. The use of any replacement thermostat other than that meeting the manufacturer's specifications will result in engine overheating! Use only thermostats meeting the specifications of Ford part number E5TZ-8575-C or Navistar International part number 1807945-C1.

Disconnect both negative battery cables.
Drain the coolant to a point below the thermostat housing.
Remove the alternator and vacuum pump belt(s)
Remove the alternator.
Remove the vacuum pump and bracket.
Remove all but the lowest vacuum pump/alternator mounting casting bolt.
Loosen that lowest bolt and pivot the casting outboard of the engine.
Remove the thermostat housing attaching bolts, bend the hose and lift the housing up and to one side.
Remove the thermostat and gasket.

To install:

Clean the thermostat housing and block surfaces thoroughly.
Coat a new gasket with waterproof sealer and position the gasket on the manifold outlet opening.
Install the thermostat in the manifold opening with the spring element end downward and the flange positioned in the recess in the manifold.
Place the outlet housing into position and install the bolts. Tighten the bolts to 20 ft. Lbs. (27 Nm).
Reposition the casting.
Install the vacuum pump and bracket.
Install the alternator.
Adjust the drive belt(s).
Fill and bleed the cooling system.
Connect both battery cables.
Run the engine and check for leaks.