Put a five gallon bucket under the exhaust for 30 seconds, measure how full and double it. Ask Trans-Atlantic diesel how much water should be flowing. Check your exhaust elbow for restrictions to water or exhaust gas flow. Either restriction can cause overheating. Also check your exhaust hose. It can disintegrate from the inside and restrict flow. Mine did. Look for soft spots which means the hose is separating inside.

I have never seen an eight vane impeller for the 4-108. I just counted 12 vanes. My Beta Marine (which replaced my 4-108) uses the same impeller for a bigger engine.