Probable Causes & Tips
                                in order of probability


1.  Blocked or Clogged Sea Water Strainer
  If blockage is caused by scaling or growth - Try dropping a Bromo tablet in to clean

2.  Through hull scupper blocked up.
  Close seacock and remove hose.  Open seacock slowly to verify.  If plugged, try
  taking a fresh water hose with nozzle to break debris up - Next go overboard and scrape

3.  Raw Water Impeller - See (6) when you inspect.
Remove cover, pull impeller and inspect.  Replace if necessary, but if vanes are missing there
is a good chance they are stuck in your heat exchanger.  Open inspection plate and try to
retrieve.  I have recently done a job where the raw water pump had a "wear" plate under the
impeller that was severely worn.  The result would be either no water flow or very low water flow.
Temporarily repaired by removing the "O" ring that this particular pump had and sealed the plate.

4.  Loose/Defective Belts

5.  Thermostat
Remove thermostat and test in pot of water on stove using a baking thermometer.  Beware: If
engine overheated enough, the thermostat can be damaged by the incident and even if found
  to be bad, may not be the source of the problem.

6.  Heat Exchanger, Oil Cooler, Transmission cooler on
raw water side of system has scale, insulating the
core from cooling the fresh water or block. Crumbled
    or broken zincs are significant contributors to blockage.
  Have engine descaled by a professional which circulates through all components, versus
paying a mechanic to come and remove the unit - have it cleaned - reinstall - and hope it was
  the unit selected.  Usually, it is the Heat Exchanger, but often enough it is the transmission
  or oil cooler that is blocked restricting the necessary water flow to accomplish sufficient heat
  transfer.  This process also dissolves any zinc debris in the system, which is why they're
  removed before process is begun.

7.  Raw Water Pump Plate is worn.
If the cover has not been replaced on an older water pump, it is likely to be worn.  Depending on
severity, the void created by the wear can affect the efficiency of the pump to supply ample
cooling.  Try sanding the plate, or thoroughly cleaning the outside and flip it.  Always apply a
new gasket, as a pin hole will suck air in, again causing a reduction of raw water flow.

8.  Fresh Water side of the system has scaling
This is a good possibility if your anti-freeze mix has been diluted with city water versus distilled
water.  City water has minerals and other qualities which lend themselves to scale buildup.

      - I CAN'T PUT A SEQUENCE ON THE FOLLOWING -

A.  Too Much Anti-Freeze
  YES - Too high a concentration of anti-freeze is a problem.   Anything stronger than 50/50
  cuts down on the mixture's cooling ability.  In fact, to address problems when the sea water
  is extremely warm, you will find that by cutting the anti-freeze to a 25% mixture concentration,
  your engine operating temperature will come down 10 degrees or more.

B.  Head Gasket
  Check oil - If water or anti-freeze is present, undoubtedly you have a head gasket issue
    Fresh Water System: Losing anti-freeze but neither see it in oil or elsewhere in bilge.  It's
highly probable head gasket has a crack, and anti-freeze is exiting out exhaust.
     Raw Water Cooled Engine:  Has to be significant.  In the above case of a fresh water system,
the breech by a mere crack itself doesn't result in overheating, the resulting decrement
of coolant causes the overheating.

C.  Bad Hoses
  While a hose may look "OK" on the surface, it is common enough to have the inner lining
  collapsing at varying degrees, thus you may not visibly see restricted flow, but it's there.
  Also, carefully check hose clamps that they are tight.  A small leak that enables pump to
  get air will negate the vacuum and result with insufficient water flow.

D.  Exhaust Riser Blocked
  Much like the collapsing hose, a riser can become partially blocked restricting water flow.
  Exhaust gases mixed with salt water create a highly corrosive compound. Depending upon
  severity, the engine may have a different sound as it's not able to adequately exhaust.  A
  descaling process as we use is not applicable to perform on the exhaust riser, as should
  there be blockage, the flushing solution could get backed up and flow back into the engine
  through the exhaust manifold.   Best thing to do is open it up and clean as best as possible.
  You may find you need to replace it.
  Diesel Applications - Riser Damaged via Electrolysis or Corrosion
  We have seen a number of them where the jacket (or baffle plate) whatever you want to call it, is
  eaten out and letting the water freely flow out the exhaust.  The baffle does two things, one has the
  water spray the exhaust to cool the
  entire perimeter of the exhaust pipe
        and also cool the gases.  The other
  aspect is that it holds water back
  somewhat to allow for heat exchange,
  that if the water flows to freely, the
  exchange of heat is actually hampered.












E.  Fresh Water Pump
At the end of my list, but not necessarily the last thing that might go wrong.  If the pump is going
bad, usually you'll notice coolant in the bilge, which is a sign of the seal going bad.  If it has
broken the shaft internally, or not turning, you should be able to detect that water doesn't flow
when the thermostat should have opened by observing the coolant through the fill cap.  DO
NOT OPEN WHILE HOT ! ! !   However,  I saw a case last fall when nothing else worked, and
while there were no apparent symptoms the fresh water pump was bad - IT WAS!  The pump
operated fine while cold, then as it warmed, the pump lost some of its vacuum.

         AND LASTLY - ITS NOT GOOD NEWS!
         The old girl is probably just worn out!
Replace or rebuild - Get a mechanic first!


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