What did you use to flush the cooling system clean water or did you use a flushing agent. The flushing agent needs to be compatible with alloy block. When you flushed it did you get any scale or muck in the water.
Regards Martin.
Yes, you never mention what has come out when you cleaned the various parts of the system. Did much come out of the fins? How mucky was the flushing liquid when you drained it. Did a lot of muck come out when you did the system flush after you cleaned the radiator?
While it's important to flush the block, overheating the coolant indicates that the problem is in heat REMOVAL ie, the radiator. There are only two reasons why the heat is not being removed. First, there is not enough coolant flow through the radiator. This could be caused by blocked passages in the radiator, or a bad water pump that is not flowing enough, or there is too much flow in the bypass caused by a thermostat that is not sealing it off effectively.
Second, there is not enough heat transfer between the coolant and the air. This is due to air blockage in the fins, or mineral coating in the coolant passages.
Since you have done a lot of cleaning, we can assume for now that heat transfer in the radiator from coolant to air is not the problem. That leaves the coolant flow as the source of the overheating. Let's also assume that the radiator passages are clear and not restricting flow. That just leaves pump flow and the bypass as the remaining suspects.
You might want to pull the water pump cover to check the impeller and internal passages of the pump. Beyond that, I have no ideas on how to test it. Perhaps someone else can offer some suggestions.
I have worked on a lot of marine engines, many of which, especially older ones had raw water cooling with block bypasses to allow the engine to warm up faster while allowing flow to cool the exhaust. I have seen a lot of old engines overheat because the bypass does not seal completely when the thermostat opens, allowing a lot of cooling water to not flow through the block. My experience is that it doesn't take much bypass leakage to overheat an engine.
I can vaguely recall that there was a brick owner who had an overheating problem that I think was caused by the way the thermostat was installed in the thermostat housing. I can't remember exactly, but it may have had something to do with how a gasket was installed. Again, maybe someone more knowledgeable may be able to supply more detail.
