Another 9.8 Mercury water pump question

[rcthd]

9.8 1978 Mercury
The water pump works fine at fast speeds but at low speeds it does not work. It is the impeller going bad or something else?

Thanks
Rick

[WarrenA]

First check and make sure the water intake isn't partly plugged. If it is ok then I would say it probably is the pump, one or more of the impellers is cracked or damaged. I have an old firestone 3.6 motor that acts the same way

[Valv]

Rcthd, I will definitely replace impeller.
If it's cooling at high speeds, your pump is not pulling enough water to cool motor at low speed.

I usually replace impeller every year, in spring, it's a & 15.00 part which will cost big $$ if neglected. It doesn't take much to do it yourself, if you know little mechanical work.

Val