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Old 31-10-2010, 07:35   #16
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Thanks everyone for the suggestions. I will look for blockage and cavitation issues.

The pump is direct controlled by a gear on the shaft which mates with a gear in the engine. I don't think it is an alignment issue because the seals are fine.

The engine is a right hand rotation.

I have pulled the heat exchanger and it is clear. The exhaust elbow is new. I might just replace all the raw water hose in case the inside wall is delaminated and caving. The run from the thruhull to strainer to pump is <3'. The run to the antisiphon valve is 3' and the run from there to the engine is 2'. So the hose runs are short and straight.

The flow rate is more difficult to check because the outlet is very close to the waterline. Just by eyeball, the two engines appear to be spitting out the same amount of water. When an impeller rips, the change in water output between the two is noticeable.

What would be the signs of cavitation inside the pump housing?

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Old 31-10-2010, 08:20   #17
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... What would be the signs of cavitation inside the pump housing?
Mark
Pump cavitation may be noisy.

Cavitation in pumps may occur in two different forms: Suction cavitation, and Discharge cavitation.

Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.
An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look sponge-like.

Discharge cavitation may sound like the popping sound produced by bone joint cracking, for example by deliberately cracking one's knuckles.
A pump that has been operating under discharge cavitation conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.
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Old 31-10-2010, 10:38   #18
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To add what Gord printed is; as well on a suction cavitation the inside of the pump will start to wear. Usually starts with pitting on the inlet side. With rubber impellers it takes a lot of replacement impellers first.

Be sure to use the wire reinforced hose on the suction side.
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Old 05-11-2010, 21:40   #19
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Check that your strainer has good gaskets in it. That all connections are tight.....it might not leak water...but it will suck air....if you are not close to a source of gaskets, you may be able to extract the old one, flip it over with a schmear of silicone on it as a temporary fix.
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Old 05-11-2010, 22:06   #20
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Check that your strainer has good gaskets in it. That all connections are tight.....it might not leak water...but it will suck air....if you are not close to a source of gaskets, you may be able to extract the old one, flip it over with a schmear of silicone on it as a temporary fix.
Another excellent point!
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