I've got a neighbor with a 47 Fountain - trips - and one engine will lose coolant when running above 3500 RPM. Keep 'er below and it'll run all day long. This is a closed cooling system btw. With the cap off - it'll overflow at idle. Pressure test the system to 25psi and it'll hold pressure for hours.

No compression issues - no water in the oil - no water trails in the exhaust

Any ideas??

Thanks

Capt Ross

could it be building too much pressure and blowing it out. does it have a pressure relief? i don't have any experience with closed cooling thats why i am asking.where is the coolant going, the bilge?

I would analyze the coolant for hydrocarbons. There is a chemical test for it that most shops have. I suspect there is a head gasket or block issue that is only showing up under extreme cylinder pressures.

radiator cap? is there a overflow tank? if so is it filling up when hot?it will lower itself when cool

I got closed cooling on my Donzi Mag slow. I have the same problem. I just keep putting water in!. I have checked everything and i mean everything. You know what i think. The only thing it can be is the coolers. PS or oil. They are copper i think and one of them must have a pin hole. Worth a look. Jas

If the coolers were bad you would have coolant in the PS or oil.

oil coolers are on the raw water side

you may still have a pin hole in the cooler that passes the lake water....when the boats running above 3500 rpm the sea-water pump is building psi and forcing water into the closed cooling system....i would check the Specific gravity of the anti-freeze and see if its diluted from what it should be....if there was a head gasket problem i think you would see some air coming out of the closed cooling system as well, right??

do you have water psi gauges on the boat? when running do they go above 25psi?

I'm leaning on testing the coolant for exhaust gases - I manage a Mercedes dealership - and now that I think of it - we have some here!!!

Sometimes is takes someone else to remind me of the simple things.......

more to come....