Losing coolant above 3500 RPM
 

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

Re: Losing coolant above 3500 RPM
 

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?

Re: Losing coolant above 3500 RPM
 

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.

Re: Losing coolant above 3500 RPM
 

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

Re: Losing coolant above 3500 RPM
 

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

Re: Losing coolant above 3500 RPM
 

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

Re: Losing coolant above 3500 RPM
 

oil coolers are on the raw water side

Re: Losing coolant above 3500 RPM
 

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??

Re: Losing coolant above 3500 RPM
 

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

Re: Losing coolant above 3500 RPM
 

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....