I read somewhere that if there is less exhaust water expullsion coming out of the outdrive and white smoke coming out, the exhaust manifold is going bad. I have a Volvo Penta AQ131A and have noticed this recently. If I replace exhaust manifold, do the risers need to be replace also? I run in saltwater here in Southwest Florida and it's brutal on my engine. Could the problem just be as simple as the impeller? Also, where can I find an online manual for this motor, 1986 Volvo Penta AQ131A?