Yanmar 3ym30 impeller

[Tony Button]

As part of recent maintenance on my 2010 Beneteau First 35, I replaced the impeller on the seawater pump of my Yanmar 3YM30 diesel engine. When I took the cover off, seawater began to leak from the pump and continued while I removed and replaced the impeller. The flow was quite small, but persisted for the 15 minutes of the operation, perhaps 5 litres in all, certainly more than the capacity of the pipe from the seawater inlet. I had closed the inlet seacock in the stern lazarette, so don't know where the water was coming from. I guess the seacock could be faulty, although it feels fine. Could the water be coming from the heat exchanger on the other side of the pump ? Seems unlikely as the flow was constant and the volume seems too much. Also, is the inlet generally higher than the pump, so the water is fed by gravity, or is the water flow controlled by the pump ? Any suggestions gratefully received !

[Compass790]

Hard to give you a hard & fast answer to that.
But first thing to try is to take the seawater feed pipe off the raw water pump after closing your seacock & check to see if it's leaking.
In lots of applications the raw water pump has to suck up the water to the pump but it depends.
I don't know how big your heat exchanger & hoses are but sometimes you can be surprised by the volume of water in a system.
I'm not sure what you are concerned about but I suppose its the possibility your seacock doesnt seal properly.

[Wotname]

What Compass790 said plus - how big is your raw water strainer?

As the flow is slow, I would also be checking the siphon break (if fitted) on the raw water pipe near the exhaust mixing point (elbow?). The siphon break should allow the raw water in heat exchanger to flow out quickly unless it is blocked.

[Tony Button]

Thanks, guys, for your responses. As you say, C790, this is not necessarily a problem but will mean I should detach and plug the hoses next time I check the impeller. I guess I was surprised as I've done this job on several boats in the past and never seen this issue. The possibility that seacock is failing is more worrying; I'll have to detach the hose at the inlet and check that it's ok. Unfortunately, the boat is new to me and I don't have a reliable plumbing diagram, so I've got to do some crawling in the bilges to check the hose runs ! I'll take the opportunity to check the siphon break and strainer. I expect to do this in the next week, so I'll let you know how I get on. Thanks again for your help.

[sailinmon]

I have a 3qm30. You likely have more water in your heat exchange elbow and exhaust than you know. It's always a wise choice to double check throughulls and sea cocks, especially if you have a ball valve instead of a flanged true sea cock but unless the water continues to flow without end, it's unlikely to be related to your seacock. If that we're leaking it wouldn't stop.