freshwater cooling leak

[noclams]

We have a 2018 Open Bow 200 with a freshwater cooling system (uses coolant). The cooling reservoir is constantly going empty and if we don't keep an eye on it, the engine overheats.

Last summer we took it to our local mechanic and they couldn’t find anything obvious - unfortunately they didn’t have the time to do a deep dive. Last winter we drove 2 hours to our closest Nautique dealer. Reservoirs were backordered so it took 3 months to get anything done. They ended up replacing the reservoir twice and also replacing several (if not all) of the cooling system. They test drove the boat and pronounced it fixed.

I towed the boat home and the boat over heated on it’s first trip. The reservoir was empty….

I was hoping that maybe some air was trapped in the system so we purchased the correct coolant and filled the reservoir as needed. We’ve run the boat 4 hours and used a gallon of coolant - so I’m thinking we still have issues.

I’m headed back to the dealer and they plan to replace the cooling system (again). Does anybody have any suggestions that I could pass along to the dealer? Is this a known problem? Should they be using an air/vac system to replace the coolant?

[NautiqueJeff]

That's a weird one.

You're not seeing any coolant in the bilge? My last three boats have leaked some coolant into the bilge, and all of them have been loose heater connections at the engine. Doesn't sound like that is your issue though.

Have you checked your oil level recently? If the block is cracked, the coolant may be getting into the oil. If you check the oil level and find it to be way higher than expected, that's bad...

[noclams]

All good points - I'll look into them. I'm pretty sure the oil is OK - it's never been high over the last few years of this problem - but it can't hurt to check again. I drained the bilge into a bucket - but it's hard to tell if it contains coolant (it's sort of a mix of salt water, rain water and dirt. I plan to take the sample to the dealer - maybe they have a way to test for coolant.

[gun-driver]

Pull your plugs and see if any are super clean(steam cleaned) could be a bad head gasket/cracked head leaking into a cylinder.

[noclams]

Another great idea - thanks!

[jpwhit]

Also check trans fluid for coolant. Could be a crack in the trans cooler. If I were tracking down this issue, I would pressure test each piece of the system separately to figure out which part had the leak.

[noclams]

Thanks!

[gun-driver]

Any updates?

[noclams]

Thanks for the followup. I dropped off the boat on Thursday and should hear back early next week. I told them all the ideas listed on on this site and they were very thankful. They plan to replace the entire cooling system under warranty - I'm not sue if it will do the trick. I will be sure to post their findings ASAP.

[Ryan747400]

If the coolant leak was in the heat exchanger, it would mix with the raw water and flow out the exhaust, which could be why you arent seeing any in the bilge.

[noclams]

Good point! We're night sure if it's leaking into the bilge or not. When we pulled the boat to go to the shop, we drained the bilge water into a bucket. It was muddy and it wasn't obvious if it contained coolant. We saved it and took it to the shop - maybe they can determine if it contains coolant. They hope to be finished next week - we'll know soon.

[bturner]

I've only worked on a Mercrusier 470 that had a fresh water cooling system which was leaking. On that one it was the heat exchanger that was leaking. Found the leak by pressurizing the heat exchanger from one side of the exchanger while having a gauge attached to the other. We could see the leak initially but by the next morning there was no pressure in the exchanger. A second try with soapy water exposed the leaking areas. Mine ended up being the seals that were on either side of the exchanger.

If it's not the hoses or exchanger then as other's have stated it's got to be coming out of the block. In that case you'd expect to see something in the bilge with the amount of fluid you're losing or through the combustion chamber from a cracked cylinder wall or blown head gasket. With either of these I would expect to see a performance issue or at a minimum white smoke in the exhaust, an unusual smelling exhaust and/or a fouled plug or 2. A compression test followed by a leak down test should find either of these and is not that hard or time consuming to preform in light of all the parts being replaced.

Changing the same parts again for in some cases the third time is a case study in Einstein's definition of insanity where you keep doing the same thing and expecting different results.

[noclams]

Amen!! The problem now is that we are entering prime season. It took a week for my shop to even find a parking spot for me - and they have a HUGE lot! They promised to pressure check everything, check the block and do all the things suggested on this site - so I remain hopeful. After our first repair attempt I towed the boat home only to find the coolant reservoir empty and the problem not fixed. This time, I plan to personally test drive the boat before the 2.5 hour drive home....

[noclams]

Here's an update: The dealer checked everything out and discovered that the boat has 3 different thermostats - the one in the port exhaust manifold was bad and indicating a faulty hight temp - this caused the system to send more cooling than it actually needed and they said it eventually mess-up the system. They replaced the heat exchanger but notice that it was still not working correctly. The tech followed the hose that goes from the heat exchanger to our heater. He found that, somewhere under the steering console, the hose was pressed against a screw and it was punctured. They replaced the hose and, after a long water test, found no coolant loss. I'm guessing that this was the problem all along - but I'm happy to have a bunch of free new parts!

Thanks again for all the help!

[bturner]

First, thank you for posting the fix. Always great to hear how something was actually resolved instead of having an open thread with no apparent resolution like we seem to get too often.

A couple items surprise me on this. The number and location of the thermostats was interesting and was most likely added to the system due to the more complex nature of todays engines, I get that and why that was so difficult to find without deeper diagnosis. The leak I'm a bit puzzled with and am more willing to chalk up to poor or lazy troubleshooting. It's not like this was the first time they saw this boat for this issue. An overnight leak down test of the system would have told them there was a leak in the system somewhere, even in the heater hose. I am surprised you didn't smell something or the antifreeze didn't manage to make it's way back to your feet if it was leaking near the heater core under the helm.

Either way glad to hear they fixed your problem and you can now enjoy the rest of the summer boating.