radiator cap/ cooling circuit

[bluefoxamazone]

hello all,

I have on the top of the radiator a cap with a red plastic lever made for a pressurised cooling circuit up to 7 lbs (I think).

I was wondering is this the right radiator cap for my engine..? Is the circuit on my Cranbrook P24  presurised or do I need another cap...?

any answer is highly appreciated,

best regards,

Franky

[desoto1939]

I looked in my Atlas replacement service handbook and for your car you should be using an R-9 cap that has a 7LB rating.  From the pictures of the ones on ebay there is no relief lever only the spring inthe bottom of the cap.  Do a search on ebay and you will see the various caps.  7 lb max rating nothing higher

 

Rich Hartung

desoto1939@aol.com

 

[squirebill]

My Plymouth manual says a P24 has a pressurized cooling system but does not say what "psi" cap should be used.  

[bluefoxamazone]

Thanks for the reply Rich, this is the one I have/

 

I guess it's ok with this one?

best regards,

[kencombs]

Looks good to me.  The lever is a nice option.  It allows release of pressure without removing cap and risking burns from hot steam or water.

[Doug&Deb]

These were originally a 4lb cap but I don’t think that’s available any longer. All of the vendors sell a 7lb now. It will work fine.

[Sniper]

I went out an looked at my cap, no pressure rating I could find but it does have A4 stamped on the center rivet, I wonder if that means 4lbs?  It's in real good shape too.

[keithb7]

I am wondering about rad caps.... When the cooling system gets hot enough it makes steam. Pressure builds up inside the rad. When 4 psi is achieved the relief in the rad cap opens end steam and coolant ejects. The hotter and hotter the system becomes, the more steam it would create. Correct? So if a 10lb cap was used,  more pressure builds up before it opens.  Possibly over pressurizing the system, also allowing the engine to run hotter before the cap opens.

 

However,  if a cooling system is in top order and running very efficiently, keeping everything nice and cool, it would not be generating up much steam pressure. Correct? If eninge is running between 150-180F on the gauge consistently, running a 10 lb cap, it would make no difference? The cooling system is not making 10 psi, nor even 4 psi. Is that correct? The problem with the 10 higher psi cap factors in when the system gets very hot. When the rad and water pump cannot keep the system cool. 10 psi would be bad.

 

Wondering if I am on the right path here in my thinking.

 

Edited August 21, 2020 by keithb7

[Sniper]

As the coolant heats up, even before getting hot enough to make steam, it expands and that's what makes the pressure.  The higher the pressure the hotter the coolant can run before going to steam.

 

Thermodynamics tells us that the biggest factor in heat exchange rates is the temperature differential.  So the hotter the coolant is compared to the air flowing thru the radiator to cool it the more efficient the exchange rate gets.

 

Obviously there is a limit to the the juggling act that is temperature and pressure in a cooling system and in our case it's usually the mechanical limits of the radiator.  Too much pressure will pop something and vent, the cap is supposed to be that thing, but when you go from 4lbs to 10lbs that's a 250% increase in pressure.  So you've given the cooling system the opportunity to show it's weak points.  Now if you have a new, modern core design radiator and heater and your hoses are new then a 14-16 psi cap is probably just fine.  If you have the original, untouched honeycomb radiator like I do then I wouldn't gamble on that. 

[desoto1939]

Correction to my original posting onthe r-9 rad cap. The r-9 is a 9lb cap and not a 7lb cap.  So If you are running a lower pressure cap of 7lbs that should be fine and the lever helps to eliminate the issue of hot steam and water coming out the top of the rad when opening the rad.  You should be fine.  Never over pound rating is not good.

 

Rich Hartung

 

[Sam Buchanan]

Containing the coolant within the system in a liquid state but under higher than atmospheric pressure raises the boiling point of the coolant. Water boils at 212*F at sea level, but if contained in a closed system the boiling point can be raised considerably. The coolant remains in the liquid phase even as temp exceeds the sea level boiling point. Steam isn't present in the system until the elevated boiling point is exceeded and at that point the game is over. Pressure ramps way up drastically and the cap relieves the pressure as gaseous and liquid dihydrogen monoxide is released. 

Edited August 21, 2020 by Sam Buchanan

[kencombs]

3 hours ago, keithb7 said:

I am wondering about rad caps.... When the cooling system gets hot enough it makes steam. Pressure builds up inside the rad. When 4 psi is achieved the relief in the rad cap opens end steam and coolant ejects. The hotter and hotter the system becomes, the more steam it would create. Correct? So if a 10lb cap was used,  more pressure builds up before it opens.  Possibly over pressurizing the system, also allowing the engine to run hotter before the cap opens.

 

However,  if a cooling system is in top order and running very efficiently, keeping everything nice and cool, it would not be generating up much steam pressure. Correct? If eninge is running between 150-180F on the gauge consistently, running a 10 lb cap, it would make no difference? The cooling system is not making 10 psi, nor even 4 psi. Is that correct? The problem with the 10 higher psi cap factors in when the system gets very hot. When the rad and water pump cannot keep the system cool. 10 psi would be bad.

 

Wondering if I am on the right path here in my thinking.

 

The missing piece in that discussion is the vacuum valve, also in the cap.  When the coolant returns to ambient temp, and shrinks as it cools, it creates a vacuum in the cooling system.  Once that vacuum is deep enough, the valve in the cap will allow air it enter.  Now, when the coolant again comes up to operating temperature it expands.  That expansion creates pressure. The amount of pressure depends on several things, how much air was admitted?  That is controlled by both the difference in running and ambient temp and the valve itself.  And, of course, the operating temp which determines the amount of expansion.  Ideally, there would never be any pressure as a result of steam forming in a normally operating system.

 

But, blockage of circulation paths in the engine due to rust and scale can result in hot spots that cause steam to form, even when the overall temp is below the boiling point.  Not much steam, but enough to cause the cap to relieve.  Steam is about 1600 times the volume of the water source!

[Sharps40]

Stant makes a 4 psi cap.   Check for fit.   If you're service manual calls for 4 then 7 is too high.  Limiting factor is the water pump seals.   Too high and you risk pushing past the shaft seal. 

 

Brass works also has a universal fit 4 psi cap.   

 

Many other sources,  acquiring should be no issue. 

 

Lever caps are wonderful leakers.  At the wrong time.  I avoid them.  

Edited August 21, 2020 by Sharps40

[Sniper]

Modern versions of our water pumps use a different seal design compared to the original, I wonder if that affects the leaky seal issue?

[bluefoxamazone]

thanks for the usefull toughts and suggestions my friends! Not finding an exact number (4 or 7 psi?) was the origin of my question. The cap I use right now does its job, I have no leakage there. But having a freeze plug at the side of the engine that starts to weep made me think of a maybe too high pressurized system. Altough I must admit that after I flushed and cleaned the cooling circuit last year with acid, this was the only freezeplug that we didn't replace.... So this one will be removed and replaced coming winter. Other than that the car runs like a charm. Does not overheat even when it is 35°C outside.

Take care, keep 'em rolling and... stay healty

greetings from Belgium!!

Franky

[greg g]

Take a good look at your radiator.  If the overflow vent is at the top of the expansion tank, above the cap, it is a zero psi setup regardless of the psi rating of the cap.  If the vent is connected to the filler neck below the cap sealing gasket, then the cap determines the pressure release psi. 

Pressure raises the boiling point of water but so does the proportion of the coolant mix.  So with a 50 50 mix and a 4 pound cap your boiling point is significantly hotternthan 212.

There is another factor to consider.  Poly propelyne glycol is not as efficient as water at transferring carrying and releasing heat. Higher surface tension of the mix reduces it ability to absorb heat from the metal it's is circulating through.  Since I do not operate my car in winter, and the garage never goes below 40 degrees, I run a 70 water, 30 antifreeze blend. This gives me the needed anti rust additive as well as having a system which operates at 170 normally and very seldom exceeds 190 regardless of traffic, outside temp or other factors.

 

If I remember when I looked it up 70 water 30 coolant gives me a 0 freezing point and 260 boiling point with zero pressure. So at 190 I still have a70 degree fusion before getting antsy.

Edited August 22, 2020 by greg g