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How do coolant expansion tanks work?

Posted by gatorbrit 
gatorbrit Avatar
Richard W
Raleigh, USA   usa
1977 MG MGB

This might sound like a dumb question, but I can't picture this in my little brain.

OK, so I have attached an expansion tank to the little tube at the top of the radiator beneath the radiator cap.

I understand that as the coolant heats up it will expand and push up the pressure valve thing in the cap and then go out down the tube into the expansion tank. That part I get.

But, when the coolant cools, the pressure will drop and the valve on the rad cap will close and it won't be able to suck the coolant back from the expansion tank back into the radiator. So it seems like this is a one way trip for the coolant.

Am I missing something here?

Many thanks
Rich




My Blue 77 B is gone after being totaled in a wreck. But the blog is still here...

http://1977mgb.blogspot.com/

mrbarry Avatar
michael barry
dover tn, USA   usa
1979 MG MGB "Moanin' Joan"
1991 Ford Bronco

my 79 has a closed system , no pressurecap on the radiator so the surge tank is pressurized with an overflow from the radiator.. in that closed system when the water in the cooling system heats up and expands it goes into the surge, when it cools off it returns to radiator,

i have never examined early systems so i am not sure about them , looks like the break between old and newer systems is 76/77 .. it appears a 77 radiator does not have a pressure cap , just an elbow that goes to that 160 dollar brass pressure vessle/expansion tank.. so is that an old radiator ? or a newer one ?

my ford truck has a plastic surge tank [ not pressurized] when the water heats and expands it pushes past a pressure cap in to the surge tank and when the radiator cools partial vacuum in the radiator sucks it back in past some kind of spring valve, i never examined it closely .maybe i will look at it and wonder..



so what i think is if you have a stock pressurized expansion tank that is where the pressure cap goes, while coupled to an older radiator with a radiator cap then the expansion tank won't work , it will accept overflow but not allow it to return, so to make it work right put a different cap on that seals above the overflow so the coolant can return. ,
on the other hand you say you installed a expansion tank , is that a plastic non pressurized one ? , then to work correctly it wants a special cap that will release water to overflow @ 15 Lbs and then have a return valve to allow the radiator to pull coolant back from that overflow tank like the 93 ford truck has.. i am going to look at that cap and system next time i have that hood up...

all this is just speculation on my part, i can only tell you how the late type III with the pressure vessel expansion tank is supposed to work..

. i am intimate with that, some years ago when i first got this jitney i was under hood with engine running and heard a strange noise from that pressure vessel turned to look and the cap blew off in my face , that was a memorable experience,
.. flash boiled face skin hanging like an ill fit fright mask ...

Gerry Avatar
Gerry Masterman
Prairieville, Louisiana, USA   usa

Yep, you are missing something. The radiator cap is a special cap that will open on pressure to let water out to the jug then will also open on vacuum to allow the water to be sucked back into the system. This type of system started back in the mid 70's. The special cap used to come with a retrofitting kit but now I suspect any new radiator cap has this feature.



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bernard lanigan
montgomery al, USA   usa
1976 MG MGB "The B"

get a cap thats correct for the expansion tank system.
yes there is a different cap design imagine the cap is a pressure releif
and a vacuum releif also so the decreased pressure in radiator due to
cool, down lets outside pressure which is slightly higher push co0olant back into radiator.

ddubois Avatar
Dave DuBois
Bremerton, WA, USA   usa

A system fitted with an expansion tank does not use a pressure cap on the radiator, just a cap that keeps it closed. The pressure cap goes on the expansion tank. think of the expansion tank as a remote header tank - that is all that it is. The system that everyone is referring to in this thread is properly called a coolant recovery system. In this system, the pressure cap remains on the radiator fill neck and the overflow tube is routed to the bottom of a catch basin located in a convenient, remote location. the catch basin has a loose fitting cap or some other overflow arrangement.

In use, the catch basin is filled about half way with coolant. As the coolant in the radiator/engine cooling passages heats up and expands, it is pushed past the pressure cap when the pressure exceeds that for which the cap is calibrated and is routed into the catch basin. After engine shut down, as the coolant in the radiator cools and contracts, the excess coolant in the catch basin is pulled back into the radiator through a centrally located vacuum release valve in the center of the pressure caps. As Gerry stated, this vacuum release valve was only on caps designed for coolant overflow systems back in the 70s when they first came out - now days, all pressure caps have the vacuum release valve built in (see photo below).
Cheers,



Dave DuBois
1953 MGTD
1966 MGB
http://homepages.donobi.net/sufuelpumps/



Edited 1 time(s). Last edit at 2008-03-20 11:47 AM by ddubois.


Member Services:
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Attachments:
Coolant Recovery 2.jpg    19.4 KB
Coolant Recovery 2.jpg

John D. Weimer Avatar
Cape Girardeau, MO, USA   usa

Great explaination Dave, textbook material and the photo "caps" it all off.

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gatorbrit Avatar
Richard W
Raleigh, USA   usa
1977 MG MGB

Dave, thanks!!! That is exactly what I was looking for. Yes, I have a coolant recovery system, not an expansion tank. I just couldn't figure out how the coolant got back in. I will take a close look at my radiator cap to make sure that it works in both directions.

Thanks again for an excellent explanation. This one really could be a nice little article in the library.

Cheers

Rich




My Blue 77 B is gone after being totaled in a wreck. But the blog is still here...

http://1977mgb.blogspot.com/

Gerry Avatar
Gerry Masterman
Prairieville, Louisiana, USA   usa

BTW, for years I have been making my own system for my MGs. I use a Nalgen bottle the same size as our washer bottles as a catch tank. I mount an old washer bottle bracket on the rad support. I then run a hose from the overflow port on the rad through a hole drilled in the top of the bottle. I run the hose all the way to the bottom of the bottle. I drill a second hole in the cap of about 1/16" for a vent. I pour an inch or so of antifreeze in the bottle to be sure the end of the hose is covered and screw the cap back on. From this point it functions just like the retrofi sytems and looks like it belongs on a MGB, something that the discount store, one size fits all systems will never do.



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JackMG Avatar
Jack Lindler
Greenville,SC, USA   usa

Pretty much all auto cooling systems have an expansion tank. On most, especially the older cars, it was an integral part (top) of the radiator. On the 77-80 MGBs, the factory moved the radiator forward, and, to make it lower to fit under the hood, replaced the integral expansion tank with an external one mounted to the passenger inner fender. Theoretically, if everything is functioning, there is never any air anywhere in the cooling system except about the top half of that expansion tank. Pressure builds up, coolant moves into the tank, cap relieves pressure if it gets too high and if that happens, after it cools there will be a little vacuum formed. This is NOT a coolant overflow recovery system, and operates much differently from one.

sailadams Avatar
Larry Adams
Atlanta, GA, USA   usa
1967 MG MGB GT "Jose The GT"
1969 MG MGB "Raggedy (Ann) Roadster"

I never knew this stuff. So -- bear with me -- if you have an old (69) radiator with the expansion tank internal, what purpose would an overflow bottle serve? I'm asking because the Moss a/c kit I'm putting in suggests adding a TR-6 coolant overflow bottle. Not a big deal, but I've been wondering, whyfor? -Larry

Gerry Avatar
Gerry Masterman
Prairieville, Louisiana, USA   usa

Even though you have vapor space in an early rad, it is still possible for this space to become full, causing your rad to overflow. An overflow tank will prevent the coolant loss that would occur here. Without one, the coolant goes on the ground. PETA people will tell you that every drop of coolant on the ground is a potential death of someone's pet



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cajuntwostepper Avatar
Dean Duhon
Youngsville, LA, USA   usa
1952 MG TD "Maggie"
1971 MG MGB "Pumpkin Nugget"
1974 MG MGB "Lil Red Riding Bonnet"

So, theoretically, Jerry, If I ride around the block till my radiator gets hot and spews bad PETA matter, I might rid our neighborhood of that pesky 'barking at all hours' dog on the next street? ummmm, be right back!

sailadams Avatar
Larry Adams
Atlanta, GA, USA   usa
1967 MG MGB GT "Jose The GT"
1969 MG MGB "Raggedy (Ann) Roadster"

Leave it to two LA boys to argue about pets. Or peta's. (Which ones cook up best, etc...)

But I suppose the Moss boys are not quite actually saying but only somewhat though rather directly implying: this a/c is going to overtax your radiator from time to time, more often than we would like to admit, and it would be best not to spew too much of this on the roadway, maybe you can even pour it back in if everything ever cools down enough to do so. Or maybe I am mistaken inferring this from the instructions, more likely... or not... or so. At least that much is clear. Thanks, boys.



ddubois Avatar
Dave DuBois
Bremerton, WA, USA   usa

Jack is correct, the header tank is in the strictest sense, an expansion tank and if you have a radiator with the fill in the top of that tank where you can see the tops of the tubes in the radiator itself and you are not putting an excessive load on the cooling system (such as an a/c system), there is very little need for a coolant recovery system or an additional external expansion tank. It is important to be able to be able to see the top of radiator tubes though if you don't use one. The reason for needing to see the tops of the tubes is so that you know when to add more coolant. The drill is on the first fill, the system will expel excess coolant. When that happens one needs to periodically check that the coolant level remains above the level of the tube tops, and add more coolant when the level is down almost to the point of exposing them. Add only enough coolant to keep the tube tops covered. On the MGAs early MGBs, with the filler neck exiting the header tank at a right angle then making a 90° turn just prior to the fitting for the cap. This makes it impossible to see the top of the tubes, thus I installed a coolant recovery system on the B (I probably would anyway because I am lazy and by having a coolant recovery coolant system, I don't have to remove the radiator cap to check the coolant level).

I can remember back in the late 50s, early 60s, when services stations would check the coolant level each time you gassed up (imagine that - service at a service station - who would have guessed!). Since the excess coolant would have been expelled, the level would be below the top of the filler and they would add water each time the car was gassed up. The result was that by the end of summer, the coolant system was about 90% water and the first hard freeze, the engine would freeze up. Back then, very few places kept mixed coolant ready to add to the system.
Cheers,



Dave DuBois
1953 MGTD
1966 MGB
http://homepages.donobi.net/sufuelpumps/


Member Services:
SU fuel pump restoration and conversion to solid state. Information and technical articles on SU fuel pumps.
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