Coolant bubbling out of the radiator cap

[F91]

Just took my 1950 Fordor 8ba on a 350 mile road trip. Car ran very well on the way over, cruising at 70 (3 spd OD). Got a little hot over in Eastern Washington as the temperature got near 100. On the way back, pulling a hill, she got hot and overheated, puked a little. Let it cool down and checked the coolant level which was still 3/4 full in the upper tank. Drove some more, but she really didn't like the heat running at the 3/4 tick mark on the temperature gauge. When I got over the pass, and the temperature dropped 20 degrees, she ran in the middle of the temp gauge all the way home (another 75 miles).
When I parked at home, I was surprised to see I still had a little bit of coolant leaking-bubbling out of the cap. radiator is an old piece of crap that I will be changing out, but my biggest concern would be a cracked block. Is it possible to have a cracked block leaking exhaust into the coolant and yet the car run fine temperature wise?
This is her, headed home-

[toby]

Don't fill the radiator so full. Try an inch over the core.

[F91]

Thanks Toby. I'd heard that before but I guess I'm too hard headed to listen.

[1931 flamingo]

Change the cap.
Paul

[F91]

Thanks Paul. I've done some research, but still don't know what pressure rating I need for the cap. Plan on getting a new cap anyway when I get the new radiator. Tanks again.

[JM 35 Sedan]

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Also, check the sealing area inside the filler neck to make sure it is in good enough condition to allow the cap (existing or new) to seal properly.
You can look for exhaust bubbles that may be visible in the coolant at top of the radiator tank. There is also an exhaust gas tester that can check for gases in the coolant at the top of the tank. My '35 fordor sedan, with an almost stock 21 stud engine, will run all day long at 55-65 mph, but as soon as I take it up to 70-75 mph (which is not very often) it will start lossing a little coolant through the check valve on the overflow tube. The cooling system is pressurized to 3-4 psi.

[toby]

A 7# radiator cap will work. Make sure it is the right depth so it seals.

[Richard in Florida]

Originally Posted by toby

A 7# radiator cap will work. Make sure it is the right depth so it seals.

Toby is right about the depth. The caps for the V8 and the 6 look about the same on the outside but the sealing surface on each is at a different depth. If you use the wrong cap you could be running with no seal at all.

[wga]

Try a O ring in the radiator neck, 1.5 inch diameter, 1/8" thick.

[F91]

Wow, all good information. Thanks everyone.

[G.M.]

Originally Posted by F91

Thanks Toby. I'd heard that before but I guess I'm too hard headed to listen.

F91 When you get your radiator and a working pressure cap fill the radiator up to the neck. As long as the pressure cap seats and works, the engine temperature don't get over 210 and you don't have a compression leak the water will stay filled for a year or more. Your system is about a 22 quart system not 16 quarts. The more water in the system, the more water flowing through the system per minute and the more air flowing through the radiator the cooler the engine runs. Driving at highway speeds the air from forward motion gives the maximum cooling. Slow driving or idling depends on the fan for cooling, this is where higher temperatures start. The hotter the day the higher the temperature gets under these conditions. Getting out of traffic and back up to speed when the temperature gets up in the 200 range will cool it back down to normal in less than 2 miles. The water going out the overflow is the result of a non working pressure cap, overheating or a compression leak.
The natural restriction of the tubes in the radiator cause the water to back up in the top tank and out the overflow tube. Ford discovered this and started to put low rated pressure caps on not to raise the boiling point but to keep ALL of the water in the system. The pressure cap allows the water to be forced through the radiator tubes blocking off the overflow tube. The pressure cap then becomes a safety valve releasing pressure through the over flow tube if the pressure in the system becomes higher than the pressure of the cap. On older radiators with no provision for a pressure cap Skip Haney makes a pressure valve that go's on the bottom end of the overflow tube.
This 3 lb valve acts much like the pressure cap and needs a good rubber seal
under the cap or it leaks. I have a lot of old Fords either with pressure caps or the 3 lb valve and never have to add water. As for the slight expansion of hot water the 4 long large diameter radiator hoses take this up. G.M.

[F91]

GM- thanks for the input. I'm leaning towards the upper tank/non-flowing tubes combined with a shot pressure cap being the culprit. I was very concerned about a crack or head gasket but the car ran well when just going down the roads temp wise. Coolant was still bubbling out but the temperature was fine. I yanked the radiator and the coolant was mostly water... Thanks again, really appreciate the advice

[G.M.]

The non flowing tubes you described are not non flowing. Brand new they restrict a full free flow of water at say just gravity flow. The same amount of water as the pumps produce just poured in the filler cap with no bottom hoses would back up and go out the over flow and just flow over the filler neck. The top of the filler tube must be sealed and the over flow sealed to force the water through the tubes. The pressure part of the cap sealing the over flow opening in the filler neck opens when the pressure gets over 4 lbs or what ever cap is installed. The faster the engine runs the faster the pumps turn. The pumps at 2.500 RPM's engine speed produce twice as much flow as when the engine turns at 1,250 RPM's. This is information from data from our water pump test machine. This is why the lose of water happens at highway speeds. G.M.

[F91]

GM- I failed to mention that about half of the tubes in the upper tank were clogged. I found out when I pulled the radiator. Anything I do will be an improvement!

[eshaver]

Originally Posted by wga

Try a O ring in the radiator neck, 1.5 inch diameter, 1/8" thick.

Hey, thanks for that tip, I may need it !