you say everything "looks" fine, but until you actually change the impeller for a new one, I suspect then you will find your problem. The only other thing would be a piece of the rubber from the old impeller lodged in the outlet of the pump, but again, you said it all looks fine. Other than those 2 ideas, the only other ideas would be a loose fan belt or the impeller somehow is no longer turning when the engine is running, and that would mean the shaft key is damaged, or the shaft itself. Good luck!!!