This is for a friend, but I'm helping him out. The generator is overheating. So far I took off the exchanger and cleaned it, Verified the thermostat was opening at the correct temp, Replaced the raw water impeller, and made sure the fluids were good. Here's what it's doing. It pumps raw water good. The exchanger is cool, but the manifold is HOT. It shuts down after 3 min. I'm thinking it's either a bubble or a bad fresh water pump. I never replaced a fresh water pump before though. How can I tell if it's working correctly. The manifold doesn't have that little bleed plug on top either. Ideas?