Raw water pump issues on Yanmar 2gm20f

[LLizzard]

I don't claim to be an expert, but it appears in the pic to me that your pump is clearly worn. There looks to be a wear pattern in the body beneath the impeller. Also, where the body compresses the impeller should be a concentric arc to create vacuum. In the pic, yours appears to go up and down like a roller coaster, probably inhibiting performance.

Also, I see a nick in the machined surface where the gasket and cover plate seal. Looks like it could have been caused by someone using a screw driver to try and pry out the impeller. NEVER remove an impeller in this manor as it damages the surface of the pump impeding proper seal. Instead you can use an impeller puller, or use pliers or vice grips to pull straight up.

If I'm you, I'd either try buying a new cover plate or smooth the outside of the existing plate and flip it. But you have that nick in the sealing surface, so perhaps you add some gasket sealant/ silicone, in addition to a new gasket. If that fails, I think a new pump is in order IMHO. Even if you get it to work marginally, it could stop anytime given the above. $200 is cheap compared to a new engine. You could also take it to a pump rebuilder to see what they think, but the new pump might be cheaper.

Good idea on the zip tie to compress the impeller. I've been using a hose clamp for 20 years to do that but a zip tie sounds easier to put on and cut off.