Recently I was asked if there was any water in our holding tanks, to which I responded "NO". It was explained to me that because we have a water heater on the boat, which travels through the heat exchanger, when we slowed down there was not enough water exchanging through the water heater there by causing the overheating at slow rpm's.
We don't overheat at 2000+ rpms because there is enough water being pulled through, which over compensates for lack of fresh water in the holding tank.
I don't know if this is related to your problem, or even if it sounds plausible. Perhaps we will be fortunate enough to have another suggestions.
Best of luck to you, Asylum
It appears that correct terms are required here. A holding tank is way different than the heat exchanger in your hot water heater.
The fresh water coolant is a closed system: manifold to fresh water coolant pump through the hot water heater and back through the thermostat. The connection to the hot water heater should be right underneath the thermostat. It has nothing to do with the fresh drinking water in your hw heater.
It is quite possible that your engine heat exchanger - the one at the aft end - is partially clogged. If you read the links I provided, you'll see that heat exchangers clog rarely through the tubes, but rather at the ports where the salt water drops out the salt onto the port. I found that when I first took out my old 2 inch HX, as well as on my newer 3 inch heat exchanger.
With the older 2 inch HX the engine when throttled back would creep up in temperature for a short time. It also would overheat when pushed to WOT, it was that way when I bought the boat, but I fixed it right away by removing the HX, and cleaning out that port after when shaking it it sounded like a mariachi band! Old zinc particles. Two different issues, but both require removing the HX. A few years later I installed the newer 3 inch HX and all is fine.
The M25 came with the small 2 inch HX. The M25XP came with the 3 inch HX.
If the OP follows the links and Burps his engine, he should be fine, assuming he also checks for leaks in his pipe connections and that his thermostat is working.
