Changing Risers and Manifolds

by Conrad & Judy Kreuter

Q: Dear Boat Talk: I’ve heard many different “rules of thumb” on when to change riser and manifolds on my V-8 I/O powered boat. Besides running warm or overheating, are there any other early symptoms to look for? JS, Hauppauge, NY.

A: Dear JS: The risers and manifolds are responsible for conducting exhaust and heat away from the motor while underway. The manifolds collect the engine exhaust as they are attached to the exhaust ports of the engine. Cooling water from the engine’s coolant systems also flows thru the manifold in separate passages and then into the riser where the cooling water is mixed with the exhaust gases and directed toward the back of the boat for discharge. Most manifolds and risers are constructed of cast iron. The manifolds are molded into the shape of a hollow log and are located on each side of your V-8 engine. Some risers are attached in the center of the manifold, others are attached to the rear of the manifold. Both mounting styles are similar to a chimney o the roof of a house. Risers extend six to eight inches above the manifold before making a sharp turn towards the rear of the boat. The riser is always above the water line to keep the outside water from flowing back into the engine. This sharp turn or angle is where most clogging problems occur.

Because of the materials used in the manifold and risers, the extreme exhaust temperatures, and the fact that most engines are cooled with salt water, corrosion and scale from the salt build up to clog the rather small cooling water passages. Unfortunately this begins to happen from day one. Flushing with fresh water after each use will help to some degree. However there is not much you can do in the way of preventative maintenance to extend the life of the manifolds and risers.

Early warning signs of failure are slowly increasing engine temperature from year to year and engine overheating especially at high RPM’s. If the engine overheats at a high RPM and returns to normal temperature at a lower RPM, it is a sure sign of restricted cooling water flow and most likely, the manifolds and risers will be defective. Occasionally this condition could also mean you have a defective sea water pump. These pumps should be changed during normal maintenance. We would recommend changing the water pump every third season and more often for heavier use.

A leaking gasket between the manifold and riser is another sign that there may be a problem. A leaking gasket can be caused by poor gasket sealing surface, failure of the gasket material, or loose mounting bolts caused by overheating. Water leaking outward from the manifold/riser joint may also mean water could be leaking into the exhaust gas passage of the manifold as well. This leak could fill one or more of the engine cylinders with water. This is a dangerous condition which could result in complete failure of the engine.

Risers and manifolds have on the average a useful life of five to eight years. Newer style manifolds and riser using different materials should provide longer useful life.