Often a sign of overfilling. Where do you fill it to when you refill? It should be just over the core fins.

Im having same issue with my 1938 Chevrolet. A local member told me not to install a thermostat, but it was not needed in California so there’s no thermostat on my engine. I just refilled the coolant and I’m getting overflow out of the overflow tube while the engine is running. Could this be overfilling? I filled it up to the top of the radiator cap. 

I am using the Evans coolant. 

Posted by: @jim-eddy

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 Could this be overfilling? I filled it up to the top of the radiator cap. 

Almost guaranteed. Fill to cover the fins and call it good. As the coolant heats it expands. If you fill to the top it has to go somewhere. I disagree with the thermostat advice you were given.

@tiny just over the fins and then let it find its own level.   This was different and the level was quite a bit low afterwards.

Other folks will have ideas when they find this thread. Hopefully we find the answer for you.

It can be many problems or combinations. How old is the radiator? I would try disconnecting the bottom hose and see how well the water flows through the radiator. With the Evans out you could then try running it with water and see how it does. You could add a small amount of soluble oil if the water works out and it doesn't freeze where you are. Instructions for tightening the packing nut are in the manual but you want it tight enough so there is a very slight drip so the shaft does not overheat.  Is your coolant foaming?

@tiny It seems i should add a thermostat then

I'm not an engineer or expert but engines were designed to work with one to keep the engine operating in a specific temperature range. Removing the thermostat is altering that temp range and is a bandaid for an issue caused by something else. Similar to using an 8v battery. If your car doesn't crank well on 6v find out why and fix it.

@tiny Ok, Thanks Tiny

@david-dunton Radiator is original to the car I believe.  No Evans here.  I’m using 50/50 Prestone green antifreeze.  The first time it happened I could not see the coolant level in the radiator.   Took quite a bit to fill it back up above the fins.  Second time the level looks ok but I did notice the coolant looking milky.  Not oily but like it has air in it.  I tightened up the packing nut some and turned the grease cup.  Will have to take another test drive and see what happens.

@jeffrey-white 

Hi Jeff

Up front, I've never actually had any of my own engines do what you are seeing.

That said, the only way this can occur is for residual heat in the block and head to continue heating the coolant in the engine after shut down (which is no longer circulating) to the point that either the liquid coolant boils to an expanding vapor... or vapor already present as entrained air/gas continues to expand as it is further heated.

If entrained air or foam is the issue, the two likely sources are air drawn in past the pump packing, or combustion gases leaking past the head gasket or through a crack. In most cases I would think air/foam would be visually apparent in the coolant in the top radiator tank.  The coolant level would have to be above the tank inlet to see this while running however so that you aren't just seeing normal turbulence, but rather actual foam and bubbles.  There are test kits made that will identify combustion gases in the coolant even in amounts small enough to be not visually apparent.  Also, some antifreeze brands are apparently better at resisting foaming than others due to their ability to facilitate the break down of the tiny bubbles in the foam, but as I said I have no experience with that.

If foaming/air entrainment or combustion gases are not the issue, and the actual coolant is boiling after shut down, the most common cause will be excessive heat being stored in the block that is moving out into the no longer circulating coolant after shut down.  This will usually be due to a lot of crud accumulated in the bottom of the water jacket in the block.  Especially at the back end.

Because this is a recent issue, my best guess is a water pump packing issue.  I hope.  🙂

@jim-eddy  @tiny

Yup.  I second what Tiny says.

Every engine with a water pump will benefit from a thermostat.

Thermosyphon engines like old John Deeres and Model T's are another thing entirely and should never have a thermostat.

I do disagree with Tiny on one point though.

Not all "engineers" have a ticket, and I would take Tiny's expertise over a lot of engineers' any day.

@ole-olson yes, I agree too and will install a thermostat. Tiny is a great engineer!