Re: Closed cooling system
Virtually all closed cooling systems tend to 'cool' best when at idle. The engine is not under load and it is producing heat at the lowest possible level. Rarely do cooling systems (boats particulary) proportionally disperse or exchange heat at the rate produced by loading and increasing engine RPM. A properly designed system should, however, maintain adequate cooling throughout the engines operating range. Extreme summertime conditions can often impact optimal cooling but in most instances, adequate circulation and flow rate dictate cooling capacity. The flow rates of both the raw and closed sides must be adequate to maintain proper cooling, it sounds like one or both must be impeded. If you've replaced the components that drive circulation (impeller, water pump) the next obvious issue lies in restricted flow. Don't assume this can only happen on the raw water side, although it is less likely. When running on muffs, is the exhast water really hot? If so, the flow restriction more probably lies on the raw water side. If cool or cold, the restriction more likely lies on the closed side.<br /><br />Navigator336 brings up a good point that often causes restriction in flow, and you can also consider that an exhaust flapper could have lodged down in the lower outdrive. I would dismantle the raw water components of the cooling system (heat exchanger, hoses, etc.) and make sure no restrictions exist. Then check exhaust risers, connections and elbows for restrictions. Remove thermostat and check flow through engine block by pumping (running) coolant (water) through the closed portion of the system. Coolant should flow easily through the system, obstructions are also potential problems in the closed portion of the heat exchanger, you'll need to check the inside of the closed coolant tube that runs through the exchanger. It is obvious that the exchanger at idle is doing the job at that rate of heat production and flow, but can not keep up with the increased production of heat. Flow is simply not increasing adequately or at all, to keep up with the increased heat load.<br /><br />I'm assuming that at one time, the system worked adequately, even though a 14" long exchanger seems small. On a smaller engine (224cid) mine is about 26 inches long at 4" diameter. The same engine, for some years outfitted with a 3" diameter exchanger, did not adequately cool at high end operation.
