Raw Water Flow Reduced After Impeller Change

All U Get · Oct 22, 2017

dkerphunter said:
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. There's a clear 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!

Is the impeller neoprene, the self-adjusting blades makes me wonder what type you have since talking with a German cruiser who has a metal impeller. I'm able to remove the hose leading to the heat exchanger and while plugging the heat exchanger opening with a rag, run the hose into a bucket. Then I could run the engine briefly to see if I have adequate water flow. I've seen hoses before the pump get soft over time and restrict water flow to the impeller. As your friend suggested air could leak in.

All U Get

All U Get · Oct 22, 2017

I follow this procedure when I change impellers. https://marinehowto.com/changing-a-raw-water-pump-impeller/

All U Get

All U Get · Oct 23, 2017

Once you determine good flow in those areas, you then can decide if it's a clogged heat exchanger or clogged mixing elbow.

All U Get

Rich Stidger · Oct 23, 2017

Since this happened and all you have done is change the impeller, I would vote for an air leak. If you had good volume and flow before changing I assume that the old impeller had all of its blades. If not, then you could still have pretty good flow but now a piece of a missing blade could be blocking the flow. Hoses don't go bad just because you changed an impeller, but they could become blocked. An air lock is a possibility.

Ralph Johnstone · Oct 23, 2017

All U Get said:
I'm able to remove the hose leading to the heat exchanger and while plugging the heat exchanger opening with a rag, run the hose into a bucket.

Run the hose underwater and you will see if you have any air getting through.

Warren Milberg · Oct 23, 2017

I also guess you are sucking air either at the pump cover gasket or the raw water hose intake connection to the pump. If you have checked for that possibility and found nothing amiss, I would re-install the old impeller that seemed to work fine. If the strong flow now returns, your new impeller is defective -- it happens a lot.

Jerry Clark H356 SV Persi · Oct 23, 2017

Had a similar experience changing my impeller last fall. I have a SpeedsealLife cover installed instead of the normal paper,gasket and cover. I had a small leak and thought it was a clogged hose to the heat exchanger as I had a couple of blades gone. I brought my air compressor to the boat the next time, blew the hose from the heat exchanger backwards, blew out the intake line and then noticed my O Ring was kinked. Installed a new O Ring, reinstalled the cover and full flow. Problem all along was the kinked O Ring, causing air to break the vacume. Make sure your seal is intact.

BigEasy · Oct 23, 2017

Sounds like you've got an air leak or an air lock. If you are getting some flow, check for a schrader type relief valve & open it to determine if air is being released from the system. You can also remove the heat exchanger cap to vent there. If all else fails, disconnect the hose going to the water heater and connect a manual hand pump to the hose using a plastic garden hose splice / union type connector. Put input hose from the manual pump in a bucket with a mixture of coolant & distilled water and pump until a solid stream of coolant returns from the fitting from which the heater hose was removed. Then reattach the full hose to the fitting. I know that many will argue that it doesn't matter which way the rubber vanes are oriented in the water pump housing; however, my yanmar service manual states that if the vanes aren't oriented properly, reduced flow will occur. Check your manual for your application. Jerry made a good point about the O ring being crimped.

Rich Stidger · Oct 23, 2017

Warren Milberg said:
I also guess you are sucking air either at the pump cover gasket or the raw water hose intake connection to the pump. If you have checked for that possibility and found nothing amiss, I would re-install the old impeller that seemed to work fine. If the strong flow now returns, your new impeller is defective -- it happens a lot.

Warren- I have never seen a defective impeller out of the box. What makes it defective, and is the defect visible? Or does it just not work even though it looks fine?

Rich Stidger · Oct 24, 2017

My experience indicates that the problem will return. Things like this just don't "fix" themselves. Hopefully you will be able to diagnose the problem with some of the suggestions in this thread. Please keep us posted.

Warren Milberg · Oct 24, 2017

Rich: There have been a number of cases where Yanmar raw water impellers have failed right out the box. If I recall correctly, the problem had to do with the central core slipping around the shaft so that the vanes didn't turn -- a quality control issue.

MitchM · Nov 8, 2017

1/ volvo MD series and the newer yan mar impellers are known to have the brass core separation problem. the vanes look fine but the hub won't lock onto the pump drive axle and very little if any pumping takes place... 2/ sometimes old double wall inlet hose from raw water strainer into raw water pump will have a collapsing interior wall which chokes the flow of h 2 o. it's worth a couple bucks to replace old inlet hoses to avoid this happening. wall collapse usually happens on suction side at higher engine speeds. the engine operating temp gradually creeps higher and higher as the interior wall deforms more and more with age .