Originally Posted by Minnewaska
Casey, I've been there and it has always turned out to be the impeller. The first gush of water was probably what was sitting in the exhaust. Pull the impeller and get a new one. It sounds as if you've unintentionally run it dry for a bit now. Clean out the interior housing of the pump with a scotch brite pad and load up the new impeller with glycerin. Overload it! You may also want to insure that your seawater strainer doesn't have an air leak.
Did not have time to work on engine yesterday (but I did get the AC to work on my 87 bmw 325e after many hours of trying to figure out the wiring when using 5 different wiring diagrams that are all supposed to be for the same car- turned out to be a bad unloader relay).
I will work on the yanmar on Friday evening. Plan to first make sure all hoses and passages are clear and then will move to the impeller. Have heard stories the impeller (even a new one) can spin on the shaft. I did load up the impeller with silicone grease prior to install. I had greased the one I removed with a water proof grease and it looked almost new when I removed it (been in service for 2 years). FWIW I had accidentaly run the engine a couple times without the seawater valve opened for several minutes about a year ago and about 6 months ago, and it apparently did not hurt the impeller- I try not to do this but mistakes happen. I use genuine yanmar impellers. I will stick to the water proof grease as this does seem to help save an impeller.