Engine Overheating NEED Help!

So here is the deal, I have a 1995 Prostar 205 with about 570 hours. We just changed the oil, transmission fluid and impeller about 8 hours ago. Basically the boat runs fine when over 25 mph and the engine temp stays around 160 but then when its in idle and at low rpms the temp goes up to 180-190 so then i kill the boat and let it cool down for a while. Then when it starts up again I can rev the engine and get water flow back through the boat and the temp goes down to 160 again. So i just added a water strainer to make sure nothing was clouged up in the raw water hose so everything should be cleared up there. I have heard that it could be the water pump which from what I understand is the housing of the impeller. O yea and the impeller is staying pretty cool much colder than the rest of the engine. What should I do? Thanks alot

Sounds like the circulating pump is going out. Is it the original? If you have water flowing through the raw water pump,and your still overheating, then the next in line is the circulating pump. Since at speed, you have good temp, it sounds like your thermostat is good.

I think Todd's right. At slow speeds the circulating water pump on the front of the engine only moves a fraction of the water it does when it's running at higher RPM. The temp of 190 is not high enough to damage anything but if the circulation is bad you might be getting more localized hot spots so it is good you are paying attention. It should drop if you just raise the RPM over 2500, in neutral, for a bit to get the circulation up when you have to idle.

That said however, if this happens shortly after you change the impeller they may be related. It is probably a good idea to check it. It will stay cool even if it is causing the engine to overheat.
All impeller type pumps, like the raw water impeller and the circulating water pump, put out a rising rate of flow as the speed increases. If you double the RPM you get quadruple the water flow. Problems show up at slower speeds.

Todd I dont know if the circulating pump is original I just bought the boat at the end of last season, I can try to find out from the original owner. So the circulating pump is different than the raw water pump? Thanks

The circulation pump is just like the water pump on a car, mounted to the front of the engine block, has the main belts driving it.
I had debris clogging my oil cooler causing engine to overheat one time.

I had that problem and mine turned out to be the ground wire on the gauge itself. Very confusing when you take off it drops to normal operating temps, gauge drops down because it was getting a voltage change at higher rpm. Just something to check into.

I'm no expert a chasing these problems but can a heat gun be used to isolate the problem? I use a $30 temp gun on my deisels to check the temp drop at the heat exchanger. It lets me know if the cores need cleaning. I also isolated another problem to a faulty gauge as the engine temp was fine but the gauge kept climbing.

You can use a heat gun to double check, but judging on the year of the boat (12yo boat) it is not unlikely that the circulating pump is shot.

It is very hard to tell if you have the factory pump on there right now. I would check the paint on the gaskets between the pump and the block to see if they have the factory black paint covering them. If they do, then it is more than likely a factory setup. If not, then it has probably been replaced before.

So it gets up to 180-190 and that is it. You may have a 180 thermostat and that is normal. if it doesnt go any higher than that then no worries.

I tend not to trust the temperature gauges. If you see steam billowing from the exhaust that is the quickest sign trouble. My boats have always gone up 10-20 degrees after pulling slalom skiers at 34 or 36mph when I stop or bring the boat back down to an idle. Is it possible that it has always done this and you didn't notice until now because you were really watching to check things after your recent service? It seems normal to me if it goes back down pretty quickly.