Raw water pump issues on Yanmar 2gm20f

[NoQuarter79]

We have been at this for a week. We have no water out of the exhaust. We started at the thru hull valve which was open, we've checked every hose, strainer is clear for that is all we did before this problem started. The impeller was good, still looked and felt brand new. We got into the heat exchanger which had small amounts of mud in it but clear enough for water to get through. We put a garden hose straight to the heat exchanger and started the engine and saw normal water flow, which gives us the impression the elbow is clear. We bought a new impeller two days ago and not even a half hour of non continuous running the impeller broke off three vanes. One was in the output of the pump, the other two were in the hose coming from the sea strainer. We are not mechanics by any means, this whole situation has been a huge learning experience but something tells us something is pushing against our pump. I have a video of us starting the motor and our pump pumping water. The hose coming from the pump to the heat exchanger pumped out a few spurts of water too. At first we over looked that but after finding those impeller pieces made their way through the intake of the pump, we are just even more confused and frustrated. We have tried everything we can think of. We tried to take the pump apart and I think we may have bent the slot in the shaft that the impeller goes into beyond repair so we're thinking a new pump. Could the pump itself pushed the impeller vanes into the hose going to the sea strainer? We are so lost and really can't afford a new pump. We've been walking the docks here in Charleston looking for extra work. We have also been on the hook this entire time. We took it over to the dock once to run the garden hose through it and it overheated, but we also had to use it a good bit to get the anchor up, but now we are back on the hook. Anything!? Please help!

[S/V Illusion]

Have you eliminated the possibility you are using the wrong or defective impellers?
On its face, the scenario you describe can best be explained by that possibility. If not, the only other cause is the pump itself.

[NoQuarter79]

It is most certainly the correct impeller. We did not install the pump backwards or upside down. The belt is in great shape and spinning. It's not possible for the impeller to spin freely because there is a bar in the center that slides into the shaft. I will try to post the video and pictures when I have some better internet.

[DeepFrz]

You said you cleaned the strainer before this happened so I would bet that you have an air leak at the strainer.

Try filling the input strainer and hose going to the pump input with water before you try starting? Also, make sure you use some glycerine on the impeller (dishwashing soap is okay as well).

Compass marine has some articles on how to replace an impellor and how to rebuild a water pump.
Rebuilding A Raw Water Pump Photo Gallery by Compass Marine How To at pbase.com

[jamhass]

Check to make sure the cam is in the correct location. Occasionally the screw holding it in place breaks, allowing it to shift. This will inhibit flow and cause impeller failure.

[NoQuarter79]

We thought it was the gasket on the sea strainer so we replaced it and still nothing. There's water when we rev the engine, but still not sufficient. We would love to rebuild this pump but we are at anchor and don't have access to many tools and we are afraid to damage it even more

[S/V Illusion]

Originally Posted by NoQuarter79

It is most certainly the correct impeller. We did not install the pump backwards or upside down. The belt is in great shape and spinning. It's not possible for the impeller to spin freely because there is a bar in the center that slides into the shaft. I will try to post the video and pictures when I have some better internet.

Yo don't mention what brand impeller so, in trying to guess the problem, I postulated a bad or improper choice as one possibility. There is abundant info on the Internet attesting to a brand which is notorious for separation allowing the hub to spin while the impeller blades do not.
I assume you already checked the Deepfrz idea of an air leak but if not, that may cause or contribute to your problem too.

[NoQuarter79]

It's a Yanmar engine, but a Johnson pump. F4b903 but the fact that it's not a Yanmar pump gives me the impression its already been replaced. It is a Johnson impeller too.

[Ziggy]

Originally Posted by NoQuarter79

It's a Yanmar engine, but a Johnson pump. F4b903 but the fact that it's not a Yanmar pump gives me the impression its already been replaced. It is a Johnson impeller too.

Yanmar uses Johnson pumps, so the pump is likely original.

You had a few reasonable suggestions for what might be the problem. Another possibility is that the raw water pump doesn't self prime. You can try putting some silicone grease over the impeller and the inside of the pump (including the cover) and see if you get the full flow. If you do, once the pump is primed, chances are it will work fine until the next time you open it up or disconnect the intake hose or open the strainer. If you don't have silicone grease, you can try some dishwashing detergent or soap, or even crisco. Just don't use petroleum based grease.

Good luck!

[Matsubob]

The 2GM is a clockwise rotation so I'm assuming the pump is built for that direction. Have you checked the thermostat? It is downstream of the water pump, attached on the front of the head. If it is blocked, you will overheat. Have you tried removing it? The fitting that encloses the thermostat has a small inlet and outlet that can get clogged easily. That should be thoroughly cleared of rust and any debris. Also, the pipe below the thermostat housing that goes into the block has a small elbow leading from the pump. This elbow can become clogged. The passages in the elbow and thermostat housing are tiny
Water enters through the pump, up to the pipe going into the block, throgh the glock and up through the head, to be expelled through the thermostat. So if the thermostat is stuck or rusted, water won't be allowed out.

[jpstevens]

I like the possibility of a leak somewhere between the thru-hull and the pump. I once had a very small leak in a hose connection between filter and AC pump. Resolving this made a huge difference in flow.
Greasing the impeller is also a good idea to insure priming.

[NoQuarter79]

We use the glycerin that came with the new impeller and we used it good. We honestly didn't think we had a thermostat because it is fresh water-cooled, but really we have no idea.

[wolfesmy]

You indicated that when revs are up it pumps but not the volume you expect. Airleak, plugged intake, wrong impeller or key not in impeller shaft. This might explain the damaged lobes on a new impeller. The woodruff key can become unseated when installing the impeller. If it was loose in the pump it would tear up an impeller.

[wimpel]

I have had similar issues with my Yanmar 3HMF. The cause of "no flow" apppeared to be backpressure air building up in the HX loop triggered by waveaction (usually happened after a wild sail trip). The solution was to disconnect the hose to the fresh water pump - start the engine until water flows - then reconnect the hose. It's a bit messy but I got it down to about 15 secs after hearing the engine not pumping water. Later I installed a vented loop over the elbow and the problem did not appear again. My impeller on the sea water pump never failed.

[lagoon410]

I had the same problem in the Bahamas. The sea strainer had some sea grass and other junk in it. I finally figured out the sea strainer was plugged with sea grass.