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It depends on what type of cooling system you have. If it's a pure raw water cooling system where raw water cools the entire engine, yes, it's a good idea to remove the t-stat before flushing.
If the engine cooling system uses a heat exchanger where the raw water is pumped in a heat exchanger then that raw water is pushed out through the manifolds and risers, there is no need to fool with the t-stat because the engine coolant, which also runs through the heat exchanger to be cooled should have permanent anti-freeze in it. This type of set up is just like an auto engine, except for having a radiator that is cooled by air flow, it has a heat exchanger cooled by raw water.
Thanks for the information before flushing raw water system with coolant I removed the thermostat
Any chance you can help me find the right fuel water separator. The one on it has no numbers at all. All that’s written on it is “ Marine electric pump fuel filter and water separator” it measures 3 1/4” tall and 3” across. No brand no numbers at all. Thanks