I’ve seen the slipping impeller twice, both aftermarket pumps from China, but still possible. They would idle fine but heat up as you revved the engine, the first one caused me fits until I turned it by hand and felt it hitting the housing. Took the pump off and found the impeller loose. Years later the second was much easier to figure out.
like I said, I'VE never seen it, but that doesn't mean it can't happen. That's good to know you've seen it. This is exactly what the forum is for. No ONE has seen it, done it all, but together we have a huge experience repository.
