Confirm the diagnosis- did not even think about an impeller until it failed...one of the more easy repairs. I would also check tge anode- many threads on here about getting the air out of the coolant system...
As mentioned, easy fix. It's either the impeller, clogged strainer, or you don't have the seacock open (or there is air in the strainer that needs primed). Let us know what you find.
Plugs: these eat them. Even if they look good! I replaced annually and carried spares.
Backwashing out impeller pieces is a bit of a pain if any vanes are missing.
If you service the impeller, if you can turn the shaft by hand the coupler is broke. Remove the fasteners around the plate the pump bolts to. Do NOT unbolt the pump from the plate face. The whole thing slides out, coupler slides on, pump shaft will slide on too once aligned. Rebolt the plate.
If you are careful…it can work. Those couplings have poor misalignment capability and you can end up chewing up couplings or worse the bearing/seal in that pump.
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Dream 'Inn III -- 2008 400 Express
PC BYC, Holland, MI
PC BYC, Holland, MI
https://www.guardiancouplings.com/products/general-purpose-couplings/gear-grip-couplings
PC BYC, Holland, MI