89 MC prostar overheat? impeller???

[sierra99]

I have just started to "play " with inboards, getting a great deal on a 89 MC prostar. I have one problem. I am overheating at idle, but when I start to move at 10MPH or faster, I then get back into the normal range. I am told that it is my impeller, and I was told to change it out every year as yearly maintanence. But How does it work? I understand that when I am underway, I am pushing water through the impeller and thus cooling the boat. But why will a bad impeller not work at idle? Does it work by vacuum? Why does a good impeller work properly at idle? Also what other spots should <br />I check while I have the impeller off??

 

[Elk Chaser]

Re: 89 MC prostar overheat? impeller???

I was told by a Merc Mechanic that you don't need to change the impeller every year. You only need to change it if your temps are higher than normal. He said he has seen 25 year old impellers that looked perfect when he disassemble them. <br /><br />The impeller is literally a pump that moves water with a pressure change. If the parts get worn the increased gap between the impeller and the housing will decrease the pressure change resulting in less water flow.<br /><br />I am not so sure that when "under way" that the water is forced through at a higher rate than the pump is capable of pumping. However the increased inlet pressure on the pump will increase the outlet pressure so when you are under way the added pressure on the inlet is helping the pump. It may just be that the pump is performing good enough at the higher rpm's to provide the required flow and is regulated by the thermostat. But at idle the pump is just not moving enough water to keep up with the heat load and the thermostat is wide open. <br /><br />Any pump has a "pump curve" where the water flow starts low at a low speed and increases on a curve to it's maximum with an increase in speed. After it has reached it's maximum flow, no matter how much faster you spin the pump the water flow will decrease on the back side of the curve. If you were to increase the inlet pressure the entire pump curve increases to a higher curve.<br /><br />Probably more than you wanted to know about pumps.