I think the thermostats on the Mercury closed cooling systems are on the antifreeze side of the motor. I know they are on the 496s.

^^^^^ Did not read any posts here but Brians above so will add the - 600 SCi is somewhat the same set up as the 496 for the front side coolant end. The t stat sits in the port side crossover with the heat exchanger mounted on top of that. The crossover is the anti freeze side as well. T stat's and water circ pumps still go bad in anti freeze sides regardless especially if you never change the coolant to new. No one likes to drain the complete system of anti freeze on Merc closed cooled systems. The coolant depletes its anti corrosive properties overtime and from heat and cooling cycles. It needs to be changed .

More info o top of - the t stat here starts to open at 143 degrees and is fully opened 100 % at 158 to 160 degrees. Anti freeze capacity should be 4 gallons total.

Ok so we finally got to the fuel cooler, and back flushed it. Bunch of debris came out of it. Took the boat for a spin, and its 90% better. So close.... Must be one more chunk of something caught in it.

So the fuel cooler is not a wide open style? It has the holes/grid/ or tubes through the cooler?

Yes it has the grid through it.