Why coolant overflow tank no work?

[Mickeyrouse]

I have a coolant mystery.
Last fall, my Westerbeke 55B engine started overheating. The coolant overflow tank was dry, I added coolant , and thought the problem solved. A week or so later same thing happened. Except this time the coolant overflow hadn’t budged from when I last checked. Aha- must be the thermostat, since I hadn’t changed it in several years. So I changed it, ran the engine, and gauge didn’t go past 140F. It’s been winter here, had a dose of COVID, so the next time I checked the engine, it ran fine- but like the other times since fall, ambient temp was around 45F. Spring comes along, getting warmer days, I run the engine- temp is maxing out. Couple days later, after the engine has cooled, I decided to check coolant at the exhaust/header tank. Bone dry, even though the overflow tank was full to the max.
Since I first installed this engine- fifteen years ago- I have always checked coolant at the overflow tank. Occasionally it’s low; I add coolant at the tank. Since the temp gauge normally reads about 185-190F, I have taken that to mean no problem, unless other issues arise like seacock left closed or stripped raw water impeller. Obvious problems, obvious solutions.
I finally determined that my loss of coolant was a slow drip on a hose at the heat exchanger. Tightened that, coolant stays where its supposed to be.
My mystery is the coolant overflow tank. Why was it showing adequate coolant, when it really wasn’t? As near as I can tell that approach has worked for years.
The hose from that tank connects to the engine at the radiator cap fitting, but there isn’t an outlet inside the cap fitting at that point. Instead there seems to be a passage to some unseen location. Thinking there could be a blockage in that passage, I connected a hose to the fitting at the radiator cap fitting and blew as hard as I could. After I regained consciousness, I decided either the other end only opens when the engine comes up to temp, or there is a serious blockage.
On the opposite of the rad. cap fitting, there is a a small tube (1/4 in ?) that the parts manual describes as the manifold air bleed tube. This runs outside the tank/manifold from the cap fitting to about where the exhaust flange is. This part is not referenced in any of the Westerbeke installation, operator’s, or technical manuals for this engine. Is this pertinent or a coincidence?
I’d really like to be able to continue my established practice of checking and adding coolant at the overflow tank that’s mounted inside a cockpit locker. I can of course check the coolant level each time I’m also checking oil levels, but that’s just one more inconvenience, and I’d prefer not getting ethylene glycol on my hands. It’s sticky, and it’s poisonous.
Surely smarter people than me are out there and can tell me what’s going on.

[bil56]

do you have a boiler connected to the engine?

no pics of the system ?

[rslifkin]

Simple things first, have you tested or replaced the pressure cap?

[gonesail]

there can be a way for coolant to cross over to the raw water circuit in the heat exchanger. maybe.

[neworleansrich]

On Westerbekes the tube connected to the coolant filler (where the "radiator cap" goes) has a very small hole going into the body of the filler. I had a Westerbeke Generator that would routinely have this hole get clogged. I had all kinds of cooling issues until I figured this out.

[gonesail]

antifreeze .. ethylene glycol .. is sticky and nasty .. but it won't poison you unless you drink it. try changing your fresh water pump if you really want to have some fun

[Lepke]

The coolant overflow tank refills the engine when the engine is off and cools down, creating a vacuum. If there's a coolant leak, the vacuum will draw air before water because it's easier.


So fix the coolant leak first. Could be leaking fitting, head gasket, heat exchanger, or out the exhaust. Coolant pressure is greater than raw water so any leak in the HE means coolant goes into the raw water side. Head gasket leak is common where the engine overheated. There the water is drawn into the cylinder during the intake stroke and goes out the exhaust. Some exhaust usually goes into the coolant. Slight leak may show as steam in the exhaust. Some coolant may show up in the oil. Leak also could be a cracked head or cylinder.
While running take off cap and look for bubbles in the coolant. There are test strips to test for combustion materials in the coolant. And running under load use a heat measuring gun to record the temp of each exhaust port. The odd cylinder usually is the leak. Sometimes there is a head gasket leak between 2 cylinders.

[Mickeyrouse]

As is so often the case the simplest solution worked: the cap wasn’t holding pressure.

[Bellinghamster]

Yep, exactly what Lepke said.