Re: how to check for bad impeller
At risk of beating the horse, I would definitely go ahead and replace it. I would say keep the old one as an emergency spare if it still looked good, but apparently an "on-the-water" swap is not possible with your merc (based on above post).
My VP impeller is right at the front of the motor...16 screws gets the bench seat loose, and another four gets me into the impeller housing.
My old one will at least get me to shore in a pinch.
Better yet, sell your old one on eBay as "slightly used"...some poor sap who doesn't frequent iBoats will buy it...
I'm kidding, of course.