Our Yanmar, like most marine diesels, has a coolant overflow tank (left top).
When checking the fluids, as I do regularly, it’s tempting to just glance at the tank and assume that if we see coolant above the “LOW” line all is well.
But that’s not necessarily so:
A friend of mine was in the habit of doing just that but even so a leak had developed in the coolant system that, over time, allowed over half the coolant in the engine to escape, which resulted in overheating.
And, worse still, by the time the alarm went off the damage was catastrophic—I think because coolant was below the overheat temperature sensor—to the point that the engine needed a rebuild and was never the same again.
But even after the disaster the overflow tank was still half full. Why?
Thanks for this one John. Indeed often overlooked – I am replacing mine ASAP. And I will change my engine pre-start check list to include the header tank too from time to time.
JL
I was trained to remove the cap on the engine itself and feel the coolant level with my finger. This is part of the pre-start checks.
The level in my header tanks has never been in a good indicator and now I know why. Time to replace those caps (both main and generator)!
We have a W40 on our Tartan 37, one time when I checked the fluid level of coolant it made a bit of a vacuum sound when taken off which made no sense to me if the coolant bottle was working. I then noticed the levels in the overflow tank did not seem to be moving with engine temperature, there ended up being a plugged passage from the neck of the rad cap that goes to the hose to the overflow, had to use a small pick to clean it out. Last year when we bought our new to us Bristol 45.5 which has a large Yanmar engine in it I noticed the levels were not moving on the coolant tank either and believe it or not that small hole in the neck of the cap was also plugged! So make sure your overflow has movement in it when the engine warms and cools.
Hi Richard,
Wow, I have not seen that. Thanks for bringing it up, and particularly for pointing out that if the coolant system is pulling a vacuum as it cools we need to dig in and find out what’s wrong.
This was enlightening. And while I can easily find the radiator cap for my Yanmar 3GM30F to purchase, where does one find only the coolant overflow tank cap? I’m finding the tanks easily, but not just the cap.
Hi Brian,
Maybe I was not clear, there is no reason I can think of to replace the coolant overflow cap. Or are you trying to add a tank to an engine where it’s missing? If so, I think the cap would come with the tank.
Ah, OK, I was misunderstanding John. I thought the header tank cap was for the coolant overflow container and that it was prone to malfunctioning. I call the engine coolant cap the “radiator cap.” Thanks for clearing that up. (I do replace the radiator cap every three or four years.)
Hi Brian,
That makes sense, I have been known to call it a “radiator cap” too.
I had to change the exhaust manifold on my Yanmar 4JH2E some time ago and for the life of me I can’t remember if the manifold came with a cap or not so it’s possible I reused the old cap. I haven’t had any problems but thanks to your tip I’m going to get another just to be sure.
I never considered this as a failure mechanism for the coolant system. My Westerbeke’s cap is over 40 years old and still working fine…if I get a spare, that cap will likely fail as soon as it knows there’s a spare nearby!
Hi Ben,
Likely? That’s a certainty.
I suspect a leak in the connecting pipework would also cause this problem: there would be no suck-back…..
Great topic, thanks for posting this John.
I will now reveal my ignorance. I built my boat, which included installing the Beta 60 natural, along with a small header tank. My head tank cap is on level with the cap on the heat exchanger. 8 years on, my header tank is still empty. I never knew whether to fill it or how much. I regularly inspect the level in the heat exchanger and keep it topped up when cold to a level just below the header tank outlet tube. How should I proceed? How does a header take actually work?
Ignorantly Yours,
Brian on Helacious, currently in Loch Linnhe, Scotland, heading towards Stavanger.
Hi Brian,
The cap on the heat exchanger has a pressure relief valve in it, so if the heat exchanger is full of coolant, as it should be, when the engine heats up to it’s operating temp the coolant expands and the valve opens letting excess coolant flow through the tube to the overflow tank. Then when the engine is stopped and cools the coolant is sucked back into the heat exchanger through the same valve.
So I would advise topping up the heat exchanger (header tank) when the engine is cold, and also filling the overflow tank to the “low” marker, as shown in the pic in the tip. Then run the engine up to temperature. At that point the level in the overflow tank should go up a bit. And then when you shut down and the engine cools, it should go back to the level you filed it to, or close.
If that does not happen and the level in the overflow tank remains the same, either the cap is bad or the tube to the overflow tank is blocked. Either way you want to fix it because otherwise the coolant system is being subjected to more pressure than it was designed for.
Thanks, John, I understand it now. Cap, line and tank appear to be working properly. Now to find the slight leak in the system…
Is it the case I assume that if you see the coolant level rise with a warm engine and then fall with it cooling down that the coolant level is ok? I assume yes since if the level was very low in the engine it would not be able to rise in the expansion tank when the engine warms up. This is what I observed on my last boat for 9 years. The problem I had to check the level in the actual engine was I could not remove the cap without the coolant in the expansion tank leaking all over.
Hi Elliott,
That’s true, but I would still remove the cap to check the level I n the heat exchange regularly. Yes, a bit spills out, but better a bit of mess than an overheated engine.
We have had a Beta 60 for ten years now. Its cooling system is different from those discussed here.
The system is normally completely sealed, except of an emergency overflow if it over pressurises.
We installed a metal header tank from Beta North Carolina. It is about a foot above the engine.
The overflow/overpressure line from this tank goes overboard
(Beta England header tank was plastic junk which would spray hot coolant and steam all around itself if the engine boils)
We never remove the filler cap on top of the engine block
We fill with coolant until the header tank is half full, idling the engine when it first appears full and topping up as air gets out.
The pressure cap on the header tank is to Beta spec. The cap on the engine is higher so it should never discharge.
Email me at if you want drawings or photos
This is an interesting discussion. What are the potential issues if at an engine’s cold state, you add antifreeze to the overflow tank, then run the engine, when it cools down there is less antifreeze in the overflow tank?
Hi John,
If that happens repeatedly, I would start looking for a leak in the coolant circuit.
The presence of a calorifier water heater adds additional complexity to this issue as, at least in my case, the coolant expansion tank is closer to the water heater (under the aft cabin bed) than the engine. In fact the engine coolant overflow bung is sealed off and (it took me too long to figure this out) the engine coolant cap should not be opened ever when there’s coolant in the system. To be specific the calorifier expansion tank and its attendant overflow tank are leveled higher than the engine if you open the engine cap you get an instant vomit of coolant all over your engine and into the bilge. Note also the the engine cap is of a higher pressure value that the cap that sits on the water heater expansion tank (13psi vs 7).
Hi Mark,
I have never had a water heater with an expansion tank, although all my boats have had water heaters. I’m guessing the reason for the expansion tank was that the heater is higher than the engine. I also think it’s a bit weird that the two caps are different. Might be worth checking what pressure the engine manufacturer calls for.