I have Perkins 4-108 vintage (89). Starts fine and plenty of power to move 42' sail boat around. The over heating starts at 2300 rpms from 180 °. to 220°. and plus, I have changed the impeller, thermostat, clean the mixing elbow and the heat exchanger. Can anyone give me some other ideas?
Have you actually checked to see that water is flowing through the engine? If there is a blockage in the block, you may get sufficient cooling for partial operation, but not for when you exceed a certain level.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
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Do you have a inlet strainer? You didn't mention it in your list of what you've done. It may be partially plugged, also perhaps your intake is grown over or somehow plugged. You should be able to hear the normal sound of exhaust if it's properly wet. Is your FW coolant level OK? If it's low you will have troubles even if your RWC system is normal.
Check your water inlet and piping BEFORE the inlet strainer. Some years ago we had a similar problem and went through lots of $$ before finding an elbow in the inlet pipe partially blocked with kelp. This allowed enough water in to cool engine at idle but not enough when under increasing power. Hope this helps!
The most common place to have restrictions, except for clogged up heat exchanger/filter/strainer etc, is in the exhaust elbow just where the water comes out to the exhaust.
Another thing that may cause problem is that pump capacity gets reduced over the years and exhaust backpressure may start making problems.