SECTION M Cooling System The  4.236  Marine  engine  is  indirectly  cooled.    This type   of   system   employs   two   separate   water   circuits. One   is   a   fresh   water   (closed)   circuit   which   circulates within the cylinder block and head, the other is a sea or river   water   (open)   circuit   which   circulates   externally round the engine.  Heat is interchanged between circuits via a heat exchanger unit. Lowline Engines A   neoprene   impeller   type   water   pump   is   used   to circulate  the  coolant  on  the  open  side  of  the  system. The raw water is drawn in through the gearbox oil cooler to the water pump.  From the pump, it is delivered to the heat  exchanger  mounted  on  the  starboard  side  of  the engine  and  then  to  the  oil  cooler  where  the  engine  oil passes  over  a  series  of  tubes  running  the  length  of  the unit.  It is through these tubes that the coolant is passed. From   the   engine   oil   cooler,   the   raw   water   is   finally discharged  through  the  exhaust  manifold  water  injection bend. Circulation   of   the   closed   fresh   water   system   is effected  by  a  centrifugal  water  pump  mounted  on  the front  of  the  cylinder  block  and  is  belt  driven  from  the crankshaft.      Water   is   drawn   from   the   header   tank mounted on the front of the engine to the cylinder block where     both     the     head     and     block     are     cooled     by thermosyphon   action.      The   water   then   passes   to   the thermostat.  When the engine is cold and the thermostat fully   closed,   the   water   bypasses   the   thermostat   to   a double entry elbow mounted on the heat exchanger via a copper  pipe.    From  the  double  entry  elbow,  the  water passes   to   the   rear   of   the   exhaust   manifold.      As   the engine   warms   up,   the   thermostat   will   start   to   open allowing  the  warm  cooling  water  to  bleed  through  the heat  exchanger  through  the  double  entry  elbow  joining up   with   the   bypass   cooling   water.      The   water   then passes through the exhaust manifold to the header tank. As  the  water  temperature  rises  to  its  allowed  maximum, the thermostat gradually opens until it is in the fully open position,  the  bypass  flow  is  cut  off  and  the  full  cooling water   flow   passes   through   the   heat   exchanger   and passes  through  the  exhaust  manifold  and  back  to  the header  tank.    The  amount  of  water  passing  through  the bypass  and  through  the  heat  exchanger  will  depend  on the opening position of the thermostat. The   header   tank   is   fitted   with   a   pressurized   finer cap. The   water   pumps   for   each   circuit   are   described individually later in this section. Standard Engines With   earlier    standard    engines,    the    header    tank mounted at the front of the engine incorporates both the engine  fresh  water  heat  exchanger  and  the  engine  oil cooler.  A pressurized filler cap is fitted. The coolant on the open side of the system is drawn in   through   the   gearbox   oil   cooler   to   the   water   pump. From  the  water  pump,  it  is  delivered  to  the  combined heat exchanger and engine oil cooler in the header tank where fresh water and oil in their respective compartments  pass  over  a  series  of  tubes  running  the length  of  the  unit.    It  is  through  these  tubes  that  the coolant on the open side is passed.  From the combined heat exchanger and oil cooler.  the water is then passed to  the  water  cooled  exhaust  manifold  and  is  then  finally discharged overboard. Circulation   of   the   closed   fresh   water   system   is effected  by  a  centrifugal  water  pump  mounted  on  the front  of  the  cylinder  block  and  is  belt  driven  from  the crankshaft.    Water  is  drawn  from  the  combined  header tank   heat   exchanger   and   is   delivered   to   the   cylinder block   where   both   the   head   and   block   are   cooled   by thermosyphon   action.      After   circulation,   the   water   is finally   discharged   at   the   front   of   the   cylinder   head through  the  thermostat  housing  to  the  header  tank/heat exchanger. The   water   pumps   for   each   circuit   are   described individually later in this section. The   operating   temperatures   of   the   closed   water system  at  the  outlet  should  be  in  the  region  of  190^oF (88^oC). A  thermostat  is  fitted  in  the  water  outlet  connection, which   enables   the   engine   to   reach   its   most   efficient working   temperature   in   the   shortest   possible   time   by restricting coolant flow.  When the correct temperature is reached,   the   thermostat   valve   opens   and   allows   the water to circulate normally.  Where a pressurized system is used, the coolant temperature will be slightly higher. To  Adjust  the  Dynamo/Alternator  and  Water  Pump Belt The   belt   tension   should   be   adjusted   so   that   it   is possible to depress the belt, without undue pressure,