I have a 2004 Volvo Penta 8.1GXi-D with a DP-SM sterndrive that has corsa captains call exhaust (captains call lets the exhaust go out through either the drive or through hulls). The problem is if the exhaust goes out through the through hulls no problem engine runs normally if it goes out through the drive it will overheat. This engine has a raw water cooling system is used in fresh water and had the sea water pump impeller replaced in the spring when the problem was first observed. Changing the impeller did not correct the problem. Any ideas on what might be causing this overheat. I have taken the drive off and do not see anything blocking the exhaust.