Hi All I am restoring a Mk2 and i have a
question.

When the engine gets warm the radiator throws out the water through
the overfill tube

I was wondering should there not be a coolant overflow tank ?

I think this keeps happening when the cooling system builds up pressure
Or is there just to much fluid in the system

Thanks for your input

Ron Wesselink

 

they came like that because the discovery of the expansion tank had not yet been...discovered?

just add one, everybody else does. In the case of my S type, a 1971 Volvo 1800-E expansion bottle was added, it's a long vertical bottle, and a bracket / holder was fabricated. works fine.

If you do a search online for "coolant expansion tank", you can read about the theory of it and how to install it correctly.

What tank you choose depends on what space you have available.

 

It is possible you have the wrong radiator pressure cap.

This is a 'quirk' of the Mark 2, in that the depth of the face where the pressure cap seal butts onto, is deeper than normal. If you fit a normal cap used on more modern cars, the sealing washer may not seal on the the face as it doesn't have enough reach. Try measuring the depth of the sealing face on the radiator, and the depth of the projection on the sealing cap that holds the sealing washer. If it won't reach then the pressure cap is wrong.

Later Jaguars had expansion tanks (as did my 1980 XJ6), and where one topped up the coolant a cap was fitted that just sealed the aperture, the pressure cap was on the expansion tank. On the older cars like the Mark 2, the coolant level in the radiator was defined to leave space for some expansion within the radiator.

 

Thanks guys for the input It solved both questions

Great going I am happy

 

I always treated the tank at the top of the rad as the expansion tank. I filled the rad so the fluid just "puddles" at the bottom of the filler area.
This ensures that the rad tubes are immersed in coolant and gives lots of room for expansion.
It's not supposed to be filled all the way to the top like modern vehicles, assuming that's what you're doing.

 

Godammit! Yet another mistake I have made. Thanks for that.

 

I must say I wondered why and how guys use an add on expansion tank, if they are connected to the overflow, there is no mechanism for the coolant to get back into the radiator, so it would not be an expansion tank, simply a catch tank unless the radiator cap is modified or one that doesn't seal is fitted and a pressure cap fitted to the "expansion vessel"

To be honest I don't know the reason behind using expansion tanks in modern cars as opposed to leaving expansion room in the radiator does anyone know ?

 

the principle or theory is that when the coolant boils it will expand, and it needs a place to go, so it ends up on the ground.

with the expansion bottle, the hot expanded coolant travels to the bottle instead of to the ground. When the coolant cools, the coolant is sucked back into the radiator by vacuum action.

It works as long as you keep the expansion bottle/tank's bottom filled with about 1 inch of coolant so no air is drawn into the radiator.

 

here's a good explanation of the way it works

British Automotive

 

Hi Jose,
I know how the physics works, but how does the coolant get past the pressure cap, do you modify it so it doesn't seal on the classics, and what was the reason all modern cars now use expansion tanks, is it for operator safety as it's less likely to cover you like a geyser than the old type radiator ?

Did a little more digging, the expansion tank is more efficient at maintaining pressure and temperature hence why it is a more widely used today, but the radiator cap has 2 valves, one to allow expansion and the other opens under vacuum when the car cools down. That's the bit I was missing. So what cap do you fit to a MK2 or SType when you add an expansion tank ? Are they part no's out there for the correct caps to use ?

 

ok Jon, I understand your question with another question:

does your radiator cap have a Spring under it? If yes, when the pressure inside the radiator exceeds the rated Radiator Cap Spring Pressure, the coolant pressure pushes the bottom rubber seal of the cap upwards, compressing the spring and allowing the coolant pressure to flow out of the radiator through the bypass hole or hose on the neck of the radiator "mouth".

prior to the expansion tanks, the coolant would just exit to the ground.

now, it goes to the expansion tank or bottle where it is returned or sucked back into the radiator via vacuum when the temperature goes down.


I recall the MK-2 and S type came with 4 lbs. rating Cap. Then they went to 7 lbs. But neither car were ever fitted with expansion tanks, that change started with the 420 and on to the XJ series. (in fact, my series 3 XJ-6 has TWO expansion tanks, or an expansion tank for the expansion tank. The second one is called a "Bladder" by Jaguar).

my S type has a 7 lb. cap, it doesn't overheat, zero, nada, even in bumper-to-bumper traffic where I just keep it in Neutral and slightly accelerate it to keep the fan forcing air.

as to part numbers, I think you simply get the cap by the rated pressure, i.e., 4 lbs. or 7 lbs. or 12 lbs., etc. You can experiment with different ratings or "calibrations" to see which one works better. Each engine is different. Some run hotter, some not.

 

Hi Jose,
I think you've missed my point, I am not being clear :-
The pressure cap works normally as you say, when the pressure builds up it exits the overflow past the sprung loaded seal, my question is how does it go back into the radiator when it cools down, modern caps have 2 valves in them, one that works as the pressure relief, and the second that opens when the pressure is lower in the radiator than the expansion vessel.

So you cannot simply use a cap that was designed for the standard radiator with a radiator and expansion vessel, as you have to get one that has 2 valves and has the long neck to seat in the radiator .

 

the radiator cap is like a Thermostat, correct Jon?

as far as I know, the coolant that travelled to the expansion tank when the cap "opened", is returned to the radiator via some sort of vacuum action, but that means that the cap would need to be in the OPEN position too, so that is exactly the point where I lose it.

 

That's exactly right, so if an expansion tank is fitted to the overflow of an older system like a MK2 or S Type then the expansion tank is simply a catch tank for the water that gets pushed out if the pressure is too high, and has no way of getting back into the radiator unless a different cap is fitted that has a second valve operating in the opposite direction which allows the coolant to pass back into the radiator. Some amount of air must be left in the radiator to allow for some expansion.

It would not have to be a vacuum (negative pressure)but the pressure has to be less in the radiator than in the expansion tank. I think that on a modern system the radiator cap has a lower relief pressure than the cap on the expansion tank, the extra volume of air in the expansion tank allows for a greater expansion of water as air compresses much more than water.
In a sealed system the volume is constant as the temp rises the pressure increases Pressure /Temp = A Constant P/T=C, too much pressure can reduce circulation of the water in the radiator, so in order to keep the pressure lower there must be some extra volume needed, that is where the expansion tank comes in, but the water that goes into the expansion tank must be allowed back into the system when it cools hence the 2 way pressure cap on the radiator, I think this is why the modern systems use this method, maybe they have lower coolant quantities which make for higher pressures, or they simply find that the system is more efficient at lower pressure, somewhere in all this is the design reason for the more common use of expansion tanks. Expansion tanks were used in the 60's, the MG1100 had this as standard back then.

 

ok Jon, in both my cars, the Caps are old if not original, I don't know if they have 2-way valves in the Caps, especially the S type, its cap is very old, it only says "7 lbs." on it and that's it, and yet the system works as it is supposed to.

 

Well Jose, I thought understood the principle, maybe I have some info missing, hopefully someone on here can shed some more light on this subject. I found some other sources online that say it won't work, but your personal experience says it does, I just don't understand why.

As I say hopefully someone on the forum can explain it, I'd be really interested to understand how it works on the older car.

JeffR1 may know ?

 

Jon,

I like this explanation: British Automotive

but curiously, the article does not say anything about the 2-way valved radiator Caps.

I am puzzled too, because the way you explain it makes sense, and yet, both my cars have old, and I mean old radiator caps. Maybe these caps already have 2 valves but they don't say anything about it? I have seen the S type's expansion bottle filled when a hot engine, and seen the bottle almost empty when cold. The bottle is a clear material and you can see the green coolant inside, it's very handy.

the "almost empty" is the trick; you have to fill the expansion tank about 1/8th so there is always about a two or three inch high puddle of coolant at the bottom of the expansion tank. This prevents air from entering the radiator through the suction hose at the bottom of the tank. All my cars are like that, and the Owner's Handbook of my Ford shows it in a drawing with an explanation to always keep the level of coolant up at the bottom of the expansion tank. A Sealed system.

and there is a second version of this system, the Sealed-Unsealed system; In other words, the expansion tank has a second nipple or port at the top, in case the gush of coolant expansion is so much that it needs to flush out to the ground or through a second hose from the top port to a second tank, like in the XJ-6. But still with the same condition: keep the coolant level about 3 inches up in the expansion tank. This means there is always coolant inside the bottom hose as well, all the way to the filler mouth of the radiator, so MAYBE, that is the clue to the answer you are looking for.


Later gater!

 

As far as I know there is no such thing as a two valve radiator cap.
As the hot coolant condenses, it creates vacuum, the perceived vacuum is air pressure from the atmosphere (15.4 lbs. per square inch at sea level approximately) pushing on the coolant in the overflow reservoir.
15.4 pound per square inch easily over comes the 7 lb. rad cap and the fluid re-enters the engines coolant system through the rad cap. (it gets by the seal in other words)
This works as long as there is no air in the system.
Air in the system "compresses" (acts like a spring if you wish) and the principal behind the system fails.
This is why if there is a leak in the system, the excess fluid doesn't get sucked back into the rad from the overflow tank.
The leak in the system allows air into it as the coolant condenses back to the ambient outside temperature.

You may be thinking that the over flow tank is sealed too, so how does the weight of the atmosphere push the coolant back into the engine if the over flow tank is sealed.

When the over flow tank is initially filled it's opened to the atmosphere, so there is 15.4 pounds of air per square inch trapped in the over flow tank.
Most over flow tanks that I've come across are not sealed though, there is a small hole in the cap some where to allow atmosphere in.
In my B2200 Mazda, you can clearly see an opening.

There is also another principal at work here, heating up the engines coolant expands the coolant. (different liquids expand at different rates of course, adding anti-freeze helps to control this rate of expansion of the water in the system, but given enough heat, the 50/50 mix of coolant does expand)
This happens no matter how much pressure a designated cap is rated at.
If the coolant expands (gets too hot) to over come the spring in the rad cap, then you will loose coolant providing that the coolant level is hard up against the cap.

As the coolant begins to return to the ambient outside temperature, it condenses (shrinks in size of you wish) and if there is no air in the system, the shrinking coolant overcomes the inner "seal" on the rad cap and the excess fluid from the overflow reservoir re-enters the coolant system.

Raising the caps pressure rating (the use of a stronger spring) also controls the boiling point of the fluid.
The highest rating that I've seen is 15 lbs.
That means it's just under the maximum weight of the atmosphere at sea level.

If you were to make a rad cap with a pressure rating of 20 lbs, the pressure inside the system would go way beyond atmospheric pressure.
With a system like this, it's very unlikely that the fragile aluminum tubes (or copper in our Jags {even thinner}) would survive that pressure.

Keep in mind that the coolant would never reach that amount of pressure assuming that the cooling system is healthy.

 

Here is a link that shows the system is supposed to have a pressure cap with 2 valves

How Does The Radiator Pressure Cap Work? | Bar's Products International, Ltd.

This is taken from another site :-
Purpose of the Pressure Cap

When the pressure inside the cooling system increases above its psi rating, the pressure cap releases some of the coolant into the overflow tank. It does this by use of a spring loaded valve. Once enough coolant has exited the system to reduce the pressure, the valve closes again. This action allows the system to remain at the optimal psi while the engine is running. This is great for when the engine is running, but what happens when you turn it off?
As the engine cools down and the pressure in the system drops, the pressure cap allows the excess coolant in the overflow tank to return to the system by use of a second spring loaded valve. This second valve responds to the vacuum created in the system from the drop in pressure and pulls the coolant out of the overflow tank. Not only does this recycle the excess coolant to prevent air pockets in the radiator, but it also prevents the softer components of the cooling system, such as the hoses, from becoming crushed by the external air pressure.


Here's another forum which talks about a different radiator cap too :-
How do coolant expansion tanks work? : MGB & GT Forum : MG Experience Forums : The MG Experience


My take is leave air space in an older system for expansion as JeffR1 said in earlier post or fit an expansion tank with a different style cap. I just can't see how a standard cap if working and sealing properly can function with an expansion tank ?

 

That's an interesting article, I was not aware of a second valve, I just thought the condensing coolant simply flowed back through the main pressure valve.

The cap in my Jag is an after market one just bought at my local auto store.
The second valve is probably there, but it simply continues to function as normal because it's not being used.

I found another article too.

How Does a Radiator Pressure Cap Work?