Raw Water Flow Reduced After Impeller Change

[Howler]

Hello, I just changed the impeller and gasket in the water pump">raw water pump on our Yanmar 3GM30F. Before the service the raw water flow was great in terms of volume and frequency. After the service, it decreased to a trickle with lower frequency. I read Charlie's post on raw water flow, but am posting separately because there's such a clear "before and after" leading to doubt there are blockages upstream or downstream of the pump area.

The impeller is new and has self-adjusting blades. The belt driving the pump is properly tensioned and turning at normal speed.

Any ideas what I might have done wrong during the change leading to this reduction in flow?

A friend suggested that the system is picking up air. Any suggestions on how to test that hypothesis?

For now I plan to repeat the steps taken to see if anything turns up but I'd appreciate any suggestions. Thanks!

[boatbod]

What do you mean the "impeller has self adjusting blades"? Sounds like you might not be using an oem impeller?

[Fuss]

Originally Posted by Howler

Hello, I just changed the impeller and gasket in the raw water pump on our Yanmar 3GM30F. Before the service the raw water flow was great in terms of volume and frequency. After the service, it decreased to a trickle with lower frequency. I read Charlie's post on raw water flow, but am posting separately because there's such a clear "before and after" leading to doubt there are blockages upstream or downstream of the pump area.

The impeller is new and has self-adjusting blades. The belt driving the pump is properly tensioned and turning at normal speed.

Any ideas what I might have done wrong during the change leading to this reduction in flow?

A friend suggested that the system is picking up air. Any suggestions on how to test that hypothesis?

For now I plan to repeat the steps taken to see if anything turns up but I'd appreciate any suggestions. Thanks!

First steps, is the new gasket sealing ok?, now remove the new impeller and compare it to the old. If everything is identical, splines, number of blades, no additional notches on the sides etc, then check carefully the pump ports for debris, then put the old one back in and compare the flow.
Once you fully understand why the flow reduced, put the new one or a correct one back in.

[Sailmonkey]

It’s possible you’ve gotten a bad impeller. This happens when the hub De-bonds from the blades and spins inside of the rubber blades.

[MV Wanderlust]

This seems so simple and probably isn't the issue... was the old impeller locked into place with a woodruff key and did you make sure it's in place for the new one? If a woodruff key slipped out and is missing, the impeller would spin around the shaft and not produce much flow. Just a thought. Probably not the issue.

[rognvald]

Did you re-install the pin in the impeller housing? Otherwise, it will freewheel irregularly. I hope this helps.

[Howler]

Lots of great ideas to investigate! Can't wait to get back onboard and continue the detective work. Thank you for the posts.

[Howler]

Oh, and to answer boadbod's, "What do you mean the "impeller has self adjusting blades"? Sounds like you might not be using an oem impeller?"....

I meant that if you switch the rotation direction the pump, the rubber blades get pushed over to the proper side. I understand that some impellers are unidirectional and can potentially be installed the wrong way. Hope that clarifies.

[J Clark H356]

Could be air due to a bad seal at the gasket or could be spinning. If you don’t get enough flow the engine will heat up,and either cut off on a heat sensor or burn up. After installing a new impeller, always check to see flow is good before completing the installation. Without enough flow you are going to damage your engine.

[Howler]

Of course. I always go on the dock and watch the water spitting out of the exhaust. That's how I noticed the problem leading to this post. Thanks.

[CarlF]

I agree it sounds like air from the pump not priming correctly. I'd replace the gasket again if you have a spare. It only takes a tiny air leak to make a pump act like this - easily caused by a tiny flaw in a new gasket.

Did the impeller you took out have all of its rubber vanes? If not, one or more may have lodged in the heat exchanger which can reduce flow. If there were any vanes missing, you have to open up the end of the heat exchanger closest to the pump and fish them out. On some engines you can just take the hose off and use long tweezers.

[skipmac]

Did you count the number of blades on the old impeller when you removed it? All there? If any bits are missing the process can easily allow them to block one of the passages. Even if the flow was good before the change, when changing impellers the water in the passages can move around allowing a loose piece of impeller that was previously trapped in a "good" place to migrate to some where in the system that it can block a hole.

[jackheape]

Originally Posted by Howler

Hello, I just changed the impeller and gasket in the raw water pump on our Yanmar 3GM30F. Before the service the raw water flow was great in terms of volume and frequency. After the service, it decreased to a trickle with lower frequency. I read Charlie's post on raw water flow, but am posting separately because there's such a clear "before and after" leading to doubt there are blockages upstream or downstream of the pump area.

The impeller is new and has self-adjusting blades. The belt driving the pump is properly tensioned and turning at normal speed.

Any ideas what I might have done wrong during the change leading to this reduction in flow?

A friend suggested that the system is picking up air. Any suggestions on how to test that hypothesis?

For now I plan to repeat the steps taken to see if anything turns up but I'd appreciate any suggestions. Thanks!

Which Charlie post are you talking about? I am going through the exact same scenario right now with a Westerbeke 21. New pump, but the impeller shredded in 60 hours. That is not unusual with this new pump Westerbeke replaced the old model with and apparently the new impellers are total crap. Th new one I put in failed in an hour. Yes there was water flow and it was well lubed. I am in the process of replacing the wet exhaust hoses now. You might want to check that as well as those hoses can deteriorate from the inside and the inner wall constrict, causing a blockage. Which might have caused the impeller to destroy itself in the first place. Also check your strainer. The gaskets can go bad on there as well, letting air in. All of those things might not have been a problem before or the initial problem, but I have discovered that once you go in and upset the equilibrium of this system it can turn into a gremlin chase. Good luck!

[alley]

Had something similar happen to me a few years ago. Turned out the new impeller was made of a softer material than the old one, so the blades were more flexible. Since my engine was deep down in the bilge, the impeller could not over come the 4-5 ft of head to push the water out of the boat, and the back pressure was bending the blades back and breaking the seal around the perimeter of the pump. I verified this by disconnecting the pump discharge and connecting to a longer hose. Started the engine and raised the hose until the outflow stopped. Dropped the hose down and the flow was just fine.

Put the old impeller back in and the flow went right back to normal. Spent the next couple of weeks looking for an impeller made of the stiffer material.

Sounds like you had some other good suggestions here. Happy hunting for the root cause!

[donradcliffe]

Sounds like the new impeller is not sealing right. I'd take it out and measure the width of the blades with a dial caliper, then compare to the old impeller and the impeller cavity.

For the guy who is shredding impellers, take the input hose off the pump and make sure you have good flow from the thru hull--it could be blocked with marine growth.