cooling problem

[model 10]

At any rpm below about 1500 while in gear, the salt water flow stops at some point. Above that it works perfect. After a lot of trouble shooting all I know is, if I disconnect the hose at the output of the pump even for a second, water starts flowing thru the system again. Before you can hear it in the exhaust note, you can feel the pump start to warm up. You would think it might be the impeller, but it was good and the flow is great at higher rpm. There is no blockage in the elbow.
It is a Perkins 4-107.

[a64pilot]

Try the pump cover, it may be sucking air or worn, if worn flip it over, also the strainer cap, it too could be sucking air

[model 10]

The pump cover is as flat as I could make it. Almost no water leaks thru the pump when it's not running. I have a new wiz bang strainer and it does not leak air. The whole system is below the water line and there are no leaks.

[south4320]

Does it only happen while in gear? Is there a heat exchanger for the gearbox?

[Lepke]

The pump is sucking air from somewhere. It could be the shaft seal, or the cover. Or the impeller or gear drive is slipping on the shaft at low rpm.

[a64pilot]

Almost no water leaks? Does that mean there are some? I ask as air of course is much less dense than water, and a leak that will only let a small amount of water leak will allow a lot of air.

Did the problem start when something was changed? Like the new strainer?

[model 10]

Almost no water gets past the impeller unless it's turning and the cover fits pretty well. There are no external water leaks, I think the pump is in good shape.
You guys make it sound like an air leak, which is good news cuz I should be able to find it and it won't cost anything.

[model 10]

Originally Posted by south4320

Does it only happen while in gear? Is there a heat exchanger for the gearbox?

There is no heat exchanger for the Gear and I have never been able to get a fault in neutral. The problem happens when we motor slowly or motor sail.
When I remove the pump output hose with the engine running, there is a very brief moment when there is no water flow and then it flows as it should.