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Both our '37 Super Eights are manifesting the same problem!
Neither overheats. I have tested the water with thermometers,
but if I drive over 40mph for more than a few minutes, the water bubbles out from under the cap and sprays all over the car.
1. The overflow tubes are clear
2. I have removed the water jacket cover from the block and there is no sediment (on the convertible sedan)
3. I have removed the lower radiator hose...no sediment in the hose or in the bottom of the tank.
4. I have had the water pump on the convertible sedan professionally rebuilt with a ss shaft and permanent seals.(I thought the pump was cavitating and sucking air into the system.

When I restored the LeBaron 20 years ago I had a radiator shop 'process' the radiator....but did not watch to see if they really removed the tanks and cleaned the tubes.

....so now I am looking for relatively easy solution to a very annoying problem.

Pulling a radiator is no easy afternoon job!

I have gotten recommendations to run the engine with
a, vinegar
b, washing soda
c, Mr. Clean

Am open to other recommendations.

I have back flushed the system with 100psi and that does get a lot of gunk out.

Aren't these old cars fun!!!!...but then we can count our blessings that we don't have to be concerned about air bags blowing up in our face!!!!!!

All suggestions are appreciated.

Keep us posted on what u do to correct the problem. Based on your description it sounds like the rad is plugged up.

There are various radiator treatments on the market that is supposed to 'boil out' crud from the radiator. I havn't used any since the late 1980's. I usually used the DuPont or other well recognized name braand.

NO!!!. i am not talking about a radiator flush or treatment that is simply a pour in and drive type treatment but rather a PROCESS that involves draining the system, dumping in the treatment, running engine for maybe 5 minutess, draining out the treatment solutution, adding a nuetralizer and then restoring the original coolant to the radiator and engine. HOWEVER i mite add that results were routinely marginal or only solved the problem for maybe a year.

Easy enuf and cheap enuf to try. U mite have better reults than i have.

Remove the radiator cap. drain out enuf coolant just below the top of the core tubes. IF the top of the core tubes are visible thru the radiator cap spigot (some rads their not visiable) use a good lite to inspect to determine if they appear to be clogged or blocked or restricted by corrosion.

Unfortunately, the '37 Super Eight has an elbow, so you can not see the tubes. In the next couple days I will be able to remove the thermostat from the top tank and inspect those tubes that I can see with a mirror.

I've never tried the vinigar trick. I have a early 50's Ag radiator here i need to get to work on. IT is definately plugged up as viewed thru the cap spigot.

I will most likely try the vinigar trick but i'm guessing it probably has to set for at least a day or more. IDK.

Not sure if trying to use muratic acid is a good ideaa or not. VERY WICKED STUFF! and dangerous to use!!! I've used the muratic to to cleaan imbedded aluminium from cylinder wall scores and things like that. But those applications are very tiny amounts of acid. Just a few drops diluted with lots of water.

So i don't know how well the muratic will work or ruin a radiator of thin braass especialy in view of the volumn of acid solution that mite be required.

Why do you mention muriatic acid? Is that a component of Mr. Clean?

I have no idea if it's a component of Mr Clean or not. I don't have a bottle of Mr Clean around here to look at. I mention the muratic acid because it is often used to clean aluminum or remove extremely difiiicult crud. IT WILL EAT ALuminum so one has to be very careful.
Let me put it this way:
If the muratic will clean deeply fined cylinder heads of an air cooled engine that runs at extreme high temperatures for over 60 yearss and that are caked up to the point to where a smallchisel and hammer won't hardly break off the dirt and grease and crud that has baked on for decades THEN why would'nt it clean a radiator????
BTW, the muratic will make the heads look like brand new streight out dealer stock from the 1940's.

Why not use the classic, oxalic acid? Follow with sodium carbonate as a neutralizer. But whatever you do don't use muriatic which is a dilution of hydrochloric acid and which can rapidly dissolve the solder in your radiator. Vinegar (acetic acid) is an old time treatment and also should be followed with a neutralizer.

Search this forum for oxalic and you'll find specifics, it's been discussed often.

Thanks Owen. I'll heed your warning about the muriatic and not try it on radiators. And thanks for the tip on oxalic.

I certainly appreciate all the input and will respond back with my results.
Bob