I have had this happen on a 25ESH. Customer taught me about it, sometimes that's how you learn things. On that particular motor, the impeller had failed and pieces had gotten jammed up into the water tube. I got all that out no problem, replaced impeller, it pee'd and I was good with that. BUT...Mr. Customer knew that it wasn't peeing as hard as it did new (and all the water pump parts were just that, new) and he bought it back to me a couple times complaining. I was sick of seeing him honestly. So he asked me if it had a thermostat (and it does), asked me to remove it. By now I'm about to get mad, but decided to keep my cool...because it's possible that he "might" be right (though in my mind at the time, "not likely"). Remove 2 bolts, removed thermostat, yep there's a piece of the impeller, stuck in the passage. Reinstalled, fired it up, pees like a brand new motor again. We both learned something, albeit in my case embarrassing.
Since then every single impeller I replace if any of the vanes are missing, FIND them and count them. Should add up to 6 every time. There's been times when a little motor like a 8 or 9.9 I can't find them right off, but removal of the exhaust cover (which has a labyrinth in it) revealed a piece or two. Failure to remove them results in overheating of the exhaust cover and/or the powerhead itself depending on where the pieces are. We obviously don't want that.
