Swirl tank cap
Discussion
I've looked through the Bible and the old threads here but I can't find mention of whether the V8S pressure cap is the same as the V6 one - (you know, the one on that red sticky up thing at the front) :)
The rubber seal has gone and the cap is none too clever, so ought to get a new one.
I looked at the parts list - can anyone confirm that I need to source a [puts on glasses and peers at piece of illegible writing] 15lb Unipart GRC1110 Quinton Hazell FC53 cap?
Does it come with the rubber bit?
Cheers!
Graham
The rubber seal has gone and the cap is none too clever, so ought to get a new one.
I looked at the parts list - can anyone confirm that I need to source a [puts on glasses and peers at piece of illegible writing] 15lb Unipart GRC1110 Quinton Hazell FC53 cap?
Does it come with the rubber bit?
Cheers!
Graham
ooooooo I just cant resist this......... Is your pressure cap on the swirl pot then? Mine is on the expansion tank and a sealing cap is on the swirl pot.. Just out of interest I had a quick look through the original owners hand book and that confirms that they should be this way round.. Cant resist seeing if the which cap where debate will start up all over again (I know.. I'm a bovver causer 
).
Oh and my pressure cap is a 15lb one but its a 2.9 not a v8 so maybe not much help, sorry.
). Oh and my pressure cap is a 15lb one but its a 2.9 not a v8 so maybe not much help, sorry.
Here we go again....
MY UNDERSTANDING - Version 1
1. Engine gets hot...
2. Water gets hot and pressurises the system...
3. The Swirl pot fills up...
4. The Pressure cap on the swirl pot takes the pressure...
5. If the pressure gets high enough or expansion gets high enough...
6. Pressure cap expels hot water into the expansion tank...
7. Water level in expansion tank rises...
8. If the system is over full...
9. Expansion tank over flows without any resistance as the is no pressure cap...
STOP...Engine cools
10. Water contracts...
11. System De-pressurises...
12. Water level in Expansion tank goes down...
13. When cold level is a few inches down in the Swirl pot...
MY UNDERSTANDING - Version 2
1. Engine gets hot...
2. Water gets hot and pressurises the system...
3. The Swirl pot fills up...
4. The Cap on the swirl pot lets water straight through the tubes to the expansion tank...
5. Water level in expansion tank rises...
6. Pressure cap on the Expansion tank resists expansion...
7. If the system is over full...
8. Pressure cap releases and overflows...
STOP...Engine cools
10. Water contracts...
11. System De-pressurises...
12. Water level in Expansion tank goes down...
13. When cold, level is a few inches down in the Swirl pot...
Come on then....Shoot me down, (if I'm wrong....)
AFAIK
There is no right or wrong way. Both versions have worked for me...
They just pressurise different areas of the system at different times.
Dave
Just to liven things up a little more... Mines had two pressure caps on the car since before i bought it in Sept? 
Out of curiosity (and due to some erratic temperatures in the last month) I've now put a sealed cap on the expansion tank, and so far haven't seen any difference! Weird.
Toby
Out of curiosity (and due to some erratic temperatures in the last month) I've now put a sealed cap on the expansion tank, and so far haven't seen any difference! Weird.
Toby
Very well explained bridgdav, and excellent diagrams.
My V8S is setup as per your description 1. I have wondered though, when the system cools down, how is it able to suck water back past the rubber seal of the pressurised cap? I would expect it to work like a one-way valve.
Therefore, in theory your description 2 sounds as though that should be the correct setup.
Perhaps the positioning of the expansion tank makes a difference? Mine ('91 model) sits on the passenger side bulkhead, whereas GI's ('92 model) is attached to the swirl pot.
Still, as they say, "if it ain't broke - don't fix it" so I'll leave mine as it is for now!
My V8S is setup as per your description 1. I have wondered though, when the system cools down, how is it able to suck water back past the rubber seal of the pressurised cap? I would expect it to work like a one-way valve.
Therefore, in theory your description 2 sounds as though that should be the correct setup.
Perhaps the positioning of the expansion tank makes a difference? Mine ('91 model) sits on the passenger side bulkhead, whereas GI's ('92 model) is attached to the swirl pot.
Still, as they say, "if it ain't broke - don't fix it" so I'll leave mine as it is for now!
DarkMatter said:
My V8S is setup as per your description 1. I have wondered though, when the system cools down, how is it able to suck water back past the rubber seal of the pressurised cap? I would expect it to work like a one-way valve.
Look more carefully at the cap. There should be another small valve in the centre which acts in the opposite direction, i.e. allows water to return to the main system.
As to where the pressure cap goes, it depends on how the various small hoses are connected up. When I had an S3, the pipe from the water pump (or was it the thermostat housing? ) went to the neck of the swirl tank above the main tank. Therefore a pressure cap here would be superfluous (sp?) and a blanking cap was used. The pressure cap was on the plastic overflow bottle. Never had a problem in the worst traffic jams. Of course, if this hose went to the main part of the swirl tank, a pressure cap could be used here.
>> Edited by B16 RFF on Friday 24th June 01:14
Le TVR said:
Good diagrams Dave,
Now:
Consider the 'heat up and pressurise the system' part of each version.
Add the compressibility of water v that of air. Add the boiling point of water at the system pressure achieved by each version.
Now, which is the more efficient system??
The boiling point of the water is the same for each version, as in both versions the system pressure will increase to that at which the pressure cap operates, which is 15 psi IIRC.
Paul
[quote=bridgdav]
Having done a lot of research into this subject, version 1 is the correct way. As stated by bridgav, pressure builds up in swirl pot. Once it gets above safe opperating pressure, ie 15lb, the spring is forced back to release pressured water into expansion tank. As the engine cools or the pressure drops the spring cap shuts again. As the system is still under pressure, a vacuum is formed. The one way valve in the cap then allows water to be drawn back into the swirl pot from the expansion tank. That is another reason why the pressure cap has to be the correct length and poundage
This is the truth your honour!!!
Having done a lot of research into this subject, version 1 is the correct way. As stated by bridgav, pressure builds up in swirl pot. Once it gets above safe opperating pressure, ie 15lb, the spring is forced back to release pressured water into expansion tank. As the engine cools or the pressure drops the spring cap shuts again. As the system is still under pressure, a vacuum is formed. The one way valve in the cap then allows water to be drawn back into the swirl pot from the expansion tank. That is another reason why the pressure cap has to be the correct length and poundage
This is the truth your honour!!!
TOPTON said:
[quote=bridgdav]
Having done a lot of research into this subject, version 1 is the correct way. As stated by bridgav, pressure builds up in swirl pot. Once it gets above safe opperating pressure, ie 15lb, the spring is forced back to release pressured water into expansion tank. As the engine cools or the pressure drops the spring cap shuts again. As the system is still under pressure, a vacuum is formed. The one way valve in the cap then allows water to be drawn back into the swirl pot from the expansion tank. That is another reason why the pressure cap has to be the correct length and poundage
This is the truth your honour!!!
Yes, but the other way works just as well if the water hoses are configured appropriately.
Fantastic, so I have my caps round the right way, maybe, depending on my hoses 
. One question. Should there ever be coolant in the expansion tank?? since getting my car back from being serviced it's about half full. I don't think it should be having read this lot. If not is it as easy as pulling the hose off to drain, or do I run the risk of getting air in the system?
. One question. Should there ever be coolant in the expansion tank?? since getting my car back from being serviced it's about half full. I don't think it should be having read this lot. If not is it as easy as pulling the hose off to drain, or do I run the risk of getting air in the system?Oh no here we go again!
If the system is in good working order, both arrangements work. It is absolutely essential that which ever caps are holding pressure are air tight, and that the pipe between the two tanks doesn't leak. Sometimes you can get caps that seal OK one way round but not the other, and in this case swapping the caps can cure a problem. Sometimes the header tank neck gets damaged and you can't get a pressure cap to seal at all, so putting the pressure cap on the swirl tank stops this tank being under pressure and works around the problem.
Having said that: if the system is in good working order then putting a blanking cap on the swirl tank and a pressure cap on the header tank works better than the other way round. By 'better' I mean that the system has a higher volume and greater flexibility which increases the durability of the system in extreme conditions, and that the tidal flow between the two tanks provides a very effective secondary degas mechanism which reduces aeration problems. These benefits are not as important as having a system that holds pressure properly, and if you have a system that only seals properly with the caps one way round, then that is the right way to put them ON THAT CAR. The neck dimensions on the two tanks are different so they usually will only seal properly one way round.
If the system is in good working order, both arrangements work. It is absolutely essential that which ever caps are holding pressure are air tight, and that the pipe between the two tanks doesn't leak. Sometimes you can get caps that seal OK one way round but not the other, and in this case swapping the caps can cure a problem. Sometimes the header tank neck gets damaged and you can't get a pressure cap to seal at all, so putting the pressure cap on the swirl tank stops this tank being under pressure and works around the problem.
Having said that: if the system is in good working order then putting a blanking cap on the swirl tank and a pressure cap on the header tank works better than the other way round. By 'better' I mean that the system has a higher volume and greater flexibility which increases the durability of the system in extreme conditions, and that the tidal flow between the two tanks provides a very effective secondary degas mechanism which reduces aeration problems. These benefits are not as important as having a system that holds pressure properly, and if you have a system that only seals properly with the caps one way round, then that is the right way to put them ON THAT CAR. The neck dimensions on the two tanks are different so they usually will only seal properly one way round.
Without going into a dispute, Quote from the bible, page 146---
"The pressure cap is placed on the swirl tank and the sealing cap without any spring is placed on the expansion tank. This seems to contradict what is said in the owners manual but, as I have said, thats the way the factory says to have them-despite what the owners manuals say. The parts manual even describes them correctly so I am not sure where the confusion cam from,"
As long as the pipe connecting the two together is on the bottom of expansion bottle and there is fluid in it, water will be drawn back into the system, not air!!
Who opened this subject again
"The pressure cap is placed on the swirl tank and the sealing cap without any spring is placed on the expansion tank. This seems to contradict what is said in the owners manual but, as I have said, thats the way the factory says to have them-despite what the owners manuals say. The parts manual even describes them correctly so I am not sure where the confusion cam from,"
As long as the pipe connecting the two together is on the bottom of expansion bottle and there is fluid in it, water will be drawn back into the system, not air!!
Who opened this subject again
TOPTON said:
Without going into a dispute, Quote from the bible, page 146---
"The pressure cap is placed on the swirl tank and the sealing cap without any spring is placed on the expansion tank. This seems to contradict what is said in the owners manual but, as I have said, thats the way the factory says to have them-despite what the owners manuals say. The parts manual even describes them correctly so I am not sure where the confusion cam from,"
As long as the pipe connecting the two together is on the bottom of expansion bottle and there is fluid in it, water will be drawn back into the system, not air!!
Who opened this subject again
That's good enough for me. Stick to the way it was GI.... I never had any overheating, or coolant system problems with that configuration! Just get the correct pressure/length as suggested.
TOPTON said:
Who opened this subject again
A fairly new member
God forbid anybody asks a question that's been asked before... BTW I have pressure cap on swirly pot, plain cap on expansion tank. I certainly found, when I got the car, that it ran much cooler that way than the otehr way round (ie how it was when I bought it).
It seems to me that the hoses can be connected differently between the swirl pot and expansion tank, one method keeps the pressure between the two, the other wouldn't, so either combination may work better on your particular car.
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