I have a coolant leaking from coolant cap on tank of my 2005 525. I pressure tested cooling system and holds 15psi with no leaks but when I run boat I get coolant coming out of cap puddles on top tank then runes down side of tank into bilge. I pressure tested cap and will only hold 4psi so ordered new cap and some thing happens. So I pressure tested new cap and it also only holds 4psi when cap says should hold 15psi. But here is real mystery the other motor does not leak even when I switch caps. Booth only hold 4psi when I test them. Temp runs fine but will slowly push out coolant through cap when running. I loose about a quart every 4 hours of running. I am thinking that cap should hold 15psi like it says and when engine heats up is slowly pushing pressure out loosing coolant. Anyone ever have this happen? Could it be caused by something else?

Do you have a crack in the neck of the coolant tank?
Have you pressure checked your heat exchanger by itself?

X2 on checking the tank.


Head gasket leak causing the cooling system to over pressurize? Was the top end refresh done at the prescribed time?

How does your coolant look? Quite possibly the heat exchanger leaking. When under way, the raw water will enter the closed cooling system and over pressurize it.

I did not pressure test just the cooler but that makes sense! Coolant does not look dirty or watered down but that really would be hard to tell just looking in tank. I will pressure test radiator see if can track down.

Some times some watered down coolant will drip out of the drive when you pull it out of the water

I will check that. Thanks

Well I pressurized heat exchanger alone and did hold 20psi. But I have 40 plus psi water pressure when running hard. I pulled plugs did leak down test on engine no signs of head gasket leaking. I guess I am going to order new heat exchanger and see if it fixes issue. 1k gamble but sick of screwing around with this.

Did you check the tank for cracks yet?

Yes, I pressure checked and inspected no leaks. The coolant is coming through cap that I am sure of.