TM 10-3930-653-14&P Fig. 4-111.  Drive Belt Tension. 4-4-3-3.  THERMOSTAT A   wax   pellet   type   thermostat   is   mounted   in   the thermostat housing at the cylinder head water outlet. The  function  of  the  thermostat  is  to  control  the  flow of coolant, facilitating fast engine warm up and regulating coolant   temperature.      The   thermostat   is   designed   to open  and  close  at  predetermined    temperatures  and,  if not operating properly, should be removed and tested as described below. (1)  REMOVAL AND INSTALLATION (1)  Drain coolant partially. (2)  Disconnect upper radiator hose at water outlet. (3)  Loosen two securing  nuts  and  remove  water  outlet, gasket, and thermostat from thermostat housing. (4)  After checking thermostat, replace or reinstall, with a new housing gasket. (5)  Reinstall water outlet and tighten securing nuts. (6)  Replenish coolant and check for leaks. Fig. 4-112.  Removing Thermostat. (2)  INSPECTION A sticking thermostat will prevent the cooling system from functioning properly.  If the thermostat sticks in the open position, the engine will warm up very slowly.  If the thermostat  sticks  in  the  closed  position,  overheating  will result.  Therefore, the thermostat should be inspected to make sure that It is in good condition. (1)    Submerge  thermostat  in  hot  water  5°C  (9°F)  above temperature specified in following table. (2)  Measure lift height of valve by inserting a screwdriver marked at point about 8 mm (0.315 in) from its tip. (3)    Remove  thermostat  and  place  in  water  5°C  (9°F) below temperature stamped on frame. (4)      Under   above   condition,   valve   should   be   closed completely.  In this case, agitate water thoroughly. If  thermostat  does  not  operate  at  above  specified temperatures,  it  must  be  replaced  because  it  cannot  be repaired. Fig. 4-113.  Inspecting Thermostat. 4-59