Impeller Housing on Yanmar

There was an air leak somewhere in the system so on the advice of Gary Silver, I went straight to the impeller as the most likely problem.  "Are you sure the O ring was seated properly?"  The impeller was in good shape but I decided to change it out anyway.  However, I had NOT changed the O ring when I last changed the impeller (in the middle of the North Atlantic two months ago).  There was corrosion around the O ring so I cleaned it up, lightly sanded it, and replaced with a new O ring.  . 

Speed Seal?

 It also seems that one of the reasons that the cover was not properly seated was that a white plastic washer that goes underneath the metal washer in the center was missing--and the center raised area (around which the impeller spun) was worn. Steve Davis ("Aloha") helped me through all this and suggested that the safest thing to do was not to use the Speed Seal.  

Service Three

I greased the inside of the housing with PTF and  used a plastic tie to pull in the impeller blades so I could ease it into the impeller housing. ​Since the previous owner had left the original solid metal cover among his spare parts, I was able to use the original rather than the Speed Seal cover.

The outcome? When I started up the engine, raw water immediately rushed through the system and I had no further problems with air leaks (at least so far). ​