have a 2008 boat with twin mercruiser 496 smartcraft engines. I have concerns about age of my manifolds I believe they are the dry manifold system with stainless risers. The boat has only about 180 hours on it.The manifolds are about 12 years old now. Boat is used in salt water but always flushed out with salt away. I had a marine mechanic pull one of the manifolds and riser off to the change a bad starter. The manifolds appear to look good inside. How long do these manifolds last? I'm looking for someone who really knows what they're talking about. I have read peoples opinions on replacing after 5 yrs but looking for a experienced mercruiser mechanics answer. Boat runs fine temperatures 170 ish. What happens when manifolds go bad? How do you inspect? Can water get in engine with the dry system if they go bad or do they just start increasing engine exhaust temperatures as they rust and clog up?