Thorney Old Problem where to put the Pressure Cap

[Gerald-TVR]

I dont want to open a huge can of worms- again but I have had some thoughts (I know I know a rare occurance)

Most usual method is for the pressure cap on the swirl pot and the non pressure cap on the expansion tank. The water level stays just about an inch above or level with the exit hose at the bottom of the container and runs fine with no further additional water. This is exactly what I would expect to see and the situation I found on my car.

BUT if you are finding that the water level continually drops to a point about 1/4 to 1/3 down the swirl pot then the pressure cap (in my opinion) is suspect as it would appear NOT to allow coolant back into the swirl pot as it cools down. If the pressure cap is on the swirl pot then it must allow 'tidal flow' back from the expansion tank. If you have a tidal flow pressure cap then the caps can be either way around. If the pressure cap dosent allow the tidal flow it must be on the expansion tank.

At one point a new pressure cap was fitted to my car and the swirl pot needed constant topping up, swaping back to the old pressure cap resolved the issue

I suspect that some pressure caps allow the tidal flow (and I would guess they are likely to have the rubber seal just below the cap) and others dont. Now its been said frequently that you have to cut an extra seal and I do wonder if those caps dont allow the tidal flow.

Just some thoughts, I agree with previous posters that both variations work but I would love to investigate further and see if I'm right about the tidal flow, or lack of it.

I have spoken to Quinton Hazell tech support today and they have confirmed that both the FC52 and FC53 (normally quoted for the V6 S) do NOT support tidal flow

I now stand back and await the barrage of disagreements!

[Roy C]

Just for clarification, this is what TVR Engineering said should happen:

Edited by Roy C on Thursday 12th May 07:22

[tubbystu]

[Griffinr]

Looks to me like the wrong cap is recomended, and what is required is a recovery type.
The following thread explains the two types, not read it all but it has good pictures.

http://www.mgcars.org.uk/mgccz/technic/pressure_me...

[tozerman]

[Gerald-TVR]

Roys post with the TVR note is fine but will have to rely on the correct type of cap, tidal flow or recovery type, to be fitted.

Usefull article from the MG boys seems to confirm my thoughts

Edited by Gerald-TVR on Wednesday 25th November 16:54

[mep12345]

Norm, you after our telephone conversation the other night you had to do this didn't you I will not respond to the TVR note above that only proves they didn't have a clue and couldn't make up their mind!!! :stirring the can of worms:

[Wiz]

Griffinr said:

Looks to me like the wrong cap is recomended, and what is required is a recovery type.
The following thread explains the two types, not read it all but it has good pictures.

http://www.mgcars.org.uk/mgccz/technic/pressure_me...

Interesting read.

I also had problems with a new pressure cap but when the old one was back on - problem sorted.

[Gerald-TVR]

Hi Mark

I'm convinced its all down to the pressure cap design, would just LOVE to bottom it out and put the discusion to bed

[mep12345]

Gerald-TVR said:

Hi Mark

I'm convinced its all down to the pressure cap design, would just LOVE to bottom it out and put the discusion to bed

As I said on the phone I agree the key is the cap if you want to put it on the swirl pot as it must then relieve the vacuum as per the MG note above. However,if you put the pressure cap on the expansion pot it doesn't matter, but one without a vacuum relief is probably better to reduce air ingress paths.

If anyone wants my full explantion of how the two methods BOTH work then search for my original explanation.

Mark

[Jed-S]

The QH FC53 caps that I've used do have vacuum relief valves.

[Gerald-TVR]

Jed-S said:

The QH FC53 caps that I've used do have vacuum relief valves.

When i spoke to QH yesterday they actually said the opposite

mep12345 said:

As I said on the phone I agree the key is the cap if you want to put it on the swirl pot as it must then relieve the vacuum as per the MG note above. However,if you put the pressure cap on the expansion pot it doesn't matter, but one without a vacuum relief is probably better to reduce air ingress paths.

Totally agree and to your technial knowledge

so the saga rolls on

[Adrian@]

Memories.... I remember that this issue (as in the bulletin) was not that the caps worked or not, but that the pressure tank was blowing the unused sensor out of the bottle.
Adrian@

Edited by Adrian@ on Thursday 26th November 11:06

[clarenceboddiger]

If the pressure cap was to be put on the swirl pot and the expansion bottle fitted with the non pressure cap then the water dispersed into the expansion bottle would just evaporate to the atmosphere over a period of time

[Gerald-TVR]

Dont you just love it when a plan come together

[tozerman]

clarenceboddiger said:

If the pressure cap was to be put on the swirl pot and the expansion bottle fitted with the non pressure cap then the water dispersed into the expansion bottle would just evaporate to the atmosphere over a period of time

That's why I have a 15lb pressure cap on the swirl pot and a 13lb pressure cap on the expansion tank.
I can see this thread exploding all over again, you've done it now Norman
Cheers.....Tony..

[clarenceboddiger]

tozerman said:

clarenceboddiger said:

If the pressure cap was to be put on the swirl pot and the expansion bottle fitted with the non pressure cap then the water dispersed into the expansion bottle would just evaporate to the atmosphere over a period of time

That's why I have a 15lb pressure cap on the swirl pot and a 13lb pressure cap on the expansion tank.
I can see this thread exploding all over again, you've done it now Norman
Cheers.....Tony..

I can see the Logic in that but shouldnt the lower pressure one be on the Swirl pot

[tozerman]

clarenceboddiger said:

I can see the Logic in that but shouldnt the lower pressure one be on the Swirl pot

Oh no here we go again

[zombeh]

You could just replace the plastic tank with a brass one (not that easy to find I wouldn't have thought though, early triumph dolomites had them)

[mep12345]

tozerman said:

clarenceboddiger said:

If the pressure cap was to be put on the swirl pot and the expansion bottle fitted with the non pressure cap then the water dispersed into the expansion bottle would just evaporate to the atmosphere over a period of time

That's why I have a 15lb pressure cap on the swirl pot and a 13lb pressure cap on the expansion tank.
I can see this thread exploding all over again, you've done it now Norman
Cheers.....Tony..

Tony that is actually potentially unsafe and you should change one or other as soon as possible. The reasons for this are that to lift on over pressurising the swirl pot would have to reach 28psi i.e. the back pressure on the second cap would act on the reverse seide of the swirl pot relief valve meaning you would need 13psi to overcome this and then another 15psi to overcome the spring pressure. You will therefore be more likely to blow a jubilee clipped joint.

Just in case anyone is in any doubt to Norm's comments about professional credentials above I am the head of a Design Authority who specialises in pressurised fluid systems within the MoD, so I do know what I am talking about when discussing pressurised coolant systems. Didn't really want to post that here as could seem pretentious, but this can of worms keeps on getting my goat I am afraid.

If Adrian's comment is correct it does mak sense, but then this is really down to TVR's poor workmanship in sealing the unused sensor hole in the expansion tank. This is because as I understand it the Triumph Herald that uses this expansion chamber as OE equipment also fits a 15psi cap to it, so the TVR bodge to seal the unused sensor is the only bit that could not be up to spec.

Mark