The Offshore Voyaging Reference Site

Don’t Be Fooled By A Coolant Overflow Tank

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?

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Jean-Louis Alixant

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.

Curt Dawson

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)!

Richard Foy

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.

Brian Sanger

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.

Brian Sanger

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.)

John Cobb

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.

Ben Logsdon

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!

Richard Ritchie

I suspect a leak in the connecting pipework would also cause this problem: there would be no suck-back…..

Bob Hodges

Great topic, thanks for posting this John.

Brian Russell

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.

Brian Russell

Thanks, John, I understand it now. Cap, line and tank appear to be working properly. Now to find the slight leak in the system…