and that would of been my fourth suggestion. At 28 years, it's surprising it's still spinning. Replacing it would have been number one for me... being without heat isn't fun.
If the impeller spun OK, my guess would be dead spot on the windings.
Well, if the ratings on the old cap and the new cap on the new pump are close, you could do a quick swap of them and test the old one, to see if it starts the new motor.
Thanks @Sniper and @Scorpio64.
Bearings seemed OK, and the startup cap looked mechanically sound. It's about a 2" sealed type, not a wet electrolytic.
My curiosity is simply aimed at determining if I should have tried to replace the startup cap first (which would have been easy) before installing a new pump. Any other reason for the pump to fail would be (in my book after 28 years) cause to replace it. $335 for the new version that's three speeds, not fixed like the old one.
Curiously, so far, I seem to be getting about two degrees more heat out of the heat pump with the new Grundfos. I won't be sure about that for a while, but I'm going to monitor it. Since the new pump has three speeds I'll have to experiment to see if there's any significant difference in efficiency.