The Merc guy said to substitute what he called the "standard" nylon-hub
water pump impeller (Merc #47-89984T-3) that came with the rebuild kit
(#46-96148A) with the "heavy duty" brass-hub impeller (#47-89984T-4). It's
the same impeller, just different hub materials. He couldn't explain why,
but said it usually works with problems like I was having. Son of a gun, he
was right. I switched them out and the problem is gone. Hallelujah."

Hope that this could be of help to you.

Bill Grannis
service manager

Bill, just a warning, The metal hub impeller was the one I had that was
spinning. It is sourced from Sierra and they have (had?) a problem with the
bonding of the brass to rubber. It was RPM sensitive but I replaced it before I
had a well developed pattern. It wasn't until I actually had it in my hand that
I found the spun hub.