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.
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