issue could be a myriad of problems. When you changed the impellor we're any of the blades missing, if so did you check elsewhere in the line to find the missing pieces.
Have you checked the water inlet. Is it blocked by growth.
In the life of the motor
have you serviced or had the heat exchanger
cleaned. Calcium buildup in the exchanger can all but stop water flow in the cooling
Just a couple of things to check that may be of assistance.