Mysterious overheating

[Alohman]

We have a problem with the raw water pump/impeller.  Sometimes when the engine is started there is no water coming out the exhaust and the engine will quickly overheat.  The engine can run fine motoring out and then the problem occurs after sailing when ready to motor home.  Other times the engine starts and runs just fine.  

When this happens, the only way to get the impeller to begin working again is to disconnect the raw water intake hose from the seacock.  Loosen the cover on the impeller (which will be dry when undone).  Pour water into the disconnected hose until water runs out the impeller.  Tighten down the impeller cover and reconnect the hose to the seacock.  Start the engine and everything is just fine.  Basically, what we are doing is priming the impeller, but why?  It seems as if at certain times there is not enough suction being created to draw water in from the seacock.  Other times the engine runs fine.

What we know is this.... M25 engine, not XP.  We did not have this problem last season.  Boat has been on the hard all winter and we have installed a brand new thermostat, brand new 3" heat exchanger, brand new universal/westerbeke impeller and brand new alternator belt.  Impeller seems to spin fine and does not slip on the shaft.  We can't find any leaks in the hoses and have checked all the hose clamps. The impeller is NOT leaking from the cover.  We have blown through the raw water intake hose to verify that it is not clogged.  This problem has only appeared since we relaunched the boat after winter storage.

Any ideas on what is going on here?
Al & Vicky Lohman

[Rodney]

I have the same engine as yours and used to have the exact same problem. Made us a bit nuts for a while, trying to figure out what was going on. Then one day as we were running the impeller and diconnecting various hoses to try to figure out what the deal was, a big "piece of goo" popped out of the hose. Never had the problem again.

I'd check the entire sea water circulation path all the way through for any obstruction.

-Tranquility  '87 Hull # 290
San Diego