Exhaust Manifolds / Stern drives & In-boards

The single largest cause of engine premature engine failure in stern drive and inboards is bad exhaust systems. And is completely preventable! This applies primarily to those of that boat in salt water, however on a smaller scale the same happens in fresh water. It just takes longer.
Replace the entire exhaust system, manifolds, risers, gaskets, every three or maximum four years. It sounds drastic and expensive but it is a small cost compared to replacement of the entire engine and exhaust system. Hundreds vs. Thousands.
The manifolds and risers are cast iron and are cooled by salt water, a highly corrosive situation. As these parts age rust flakes off of the inner walls of the water jackets. These walls have thin and thick spots because of the imprecise casting process. The flakes may get blocked where they exit the riser, as they back up the riser becomes plugged and the cooling water cannot flow fast enough to cool the motor. When you check the riser you find the “plug”. Often times people try to bang the parts and knock out the blockage, some even send to radiator shops and have them acid boiled and rodded , or simply replace the riser. This stops the over heating, however it is a symptom of a more serious problem .
The inner walls of the manifolds have become thinner and thinner ( remember those rust flakes), hot exhaust gas on one side of the wall and cool water on the other. While the motor is running, it will crack. Any leakage is blown out with the exhaust and you are unaware there is a problem. When you stop the motor all the salt water in the exhaust system leaks though the crack into the motor and lies on top of the pistons. If it is one or two weeks until you start the boat again , it is rusted solid and you need a new motor and exhaust system , $5000.00 to $10,000.00 and it could have been avoided.
The manifolds cannot be visually inspected , a pressure test only tells if it is leaking now. Inspect the risers each year and replace when they show signs of clogging or excessive rust. Regardless of condition replace entire system at no more than four years, I prefer three. If you budget this cost over four year period you will find it is not that expensive. Along with regular oil changes and normal care this is the most important thing you can do to extend the life of you motor.