I replaced the impeller and removed the power head to inspect for excess exhaust pressure in cooling jacket, with no evidence of problems. If the large rubber shift adjustment inspection cover is open, the water will "pee" out the tube but when I close the inspection cover, the water stops and then begins to overheat. I wondered about back pressure, but nothing has changed in performance. It seems sometimes as if the impeller is air bound, however it has plenty of open water porting. There is no thermostat on this model.