Quote Originally Posted by blacksambo View Post
Water pump is original. But maybe I'm dense, but why would less rpm cause lower water temp. to occur? Thx
Cavitation. Been through it with the impeller on my SeaDoo. It runs fine at idle but more & more efficiency is lost as the speed increases. Basically the pump could be causing an air pocket to form in the pump restricting flow at higher rpm whilst at idle everything is working fine.

The reason that we usually don't see erosion of the pump blades is that the seals usually fail long before that occurs.

I'm not sayin that your pump is bad ... it's just one of the possibilities to consider.