I have a boat with a MerCruiser engine. Is it normal for one exhaust manifold to run very warm while the other runs cool to the touch?
MerCruiser says that this is normal. Their response is: Water coming out of the thermostat housing is not divided equally between the starboard and port side. Water takes the path of least resistance, and most will go out one side. The other side does get water, but not the same amount. Hence, one manifold (or exhaust elbow) will be cooler because of more water flow than the other side with less water flow. This should be more noticeable at lower speeds. At higher speeds, more water is flowing and both manifolds get closer in temp to the touch.
