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Re: Raw Water Cooled for 35 Years?
What really matters is the alloy used in the block, the chemical content and salinity of the raw water, and the durability of the engine.
Some have a reputation for enduring. Others not so much. I've heard that older Volvo engines in brackish to salt water will often rust through in the cooling passages, and that 25-30 years is all you can expect. I don't know if that's true or not.
But for any old raw water engine, used in salt water? You'd want to disconnect the water hoses, possibly pull the water pump or thermostat, whatever you have to do on that particular engine in order to access the cooling passages and see if they are eating into the block or not. Many are surprisingly robust.
And if the PO generally kept the raw water intake closed? Remember that raw water doesn't circulate unless it is OPEN, and whatever corrosion is going to happen, will self-limit itself if the water isn't flowing and being replaced. It very well could be fine.
Only way to really find out? Get some eyes in there, or a borescope.
I have looked at the inside passages of my 30 year old sea water cooled yanmar. What I see is white deposits and no rust or corrosion. I am thinking this is calcium carbonate precipation from the sea water.
I am hesitant to perform an acid flush of the system as all this coating may get stripped and actually increase corrosion rates. Maybe this coating is reducing corrosion of the engine block. Now if the calcium carbonate deposit gets too thick, then heat transfer will be reduced and the engine may start overheating.
I am thinking maybe do a 50/50 vinegar soak and flush once a year to keep the deposits to a minimal level, but keep them intack to protect against corrosion.