How To Understand Pressure Caps and Header Tanks

[grout20]

Excellent article there ... must have taken ages!

The "simple" Pressure Cap!?

Like they used to say about the Radio Times ... "I never knew there was so much in it"



John

[Jdwoodhou]

I think your theory about scale in the header tank is correct. I live in a hard water area so I've used deionised rather than tap water mixed with my OAT antifreeze from new (and flushed the system a couple of times) and have no scale.

[T-Cut]

Originally Posted by Dragrad

The post has been pdf'd to retain for posterity, and therfore reclaimable should you accidntally move the images' locations

Many thanks. I've just corrected one of the photos that was duplicated in error. Please would you update the PDF for me?

I suspect I'll need to revise/correct it more as comments are made. With access only to this copy, I'll amend this one. Do I then ask a mod to update the PDF at the same time?

TC

Originally Posted by Salad-Dodger

What I was wondering was how the sensor was sealed in the BMW tank. There must have been an "O" ring in there and I'd like to know the size of the hole too.

Yes, an o-ring seal was the most likely way. I believe it was intended to use a reed switch much like the one fitted in the screen wash reservoir. There's a thread somewhere showing a member's attempt to fit one into a black tank. I think the sensor fits bayonet fashion into the base.

TC

Originally Posted by Mike Noc

One thing puzzles me - what is the purpose of the lower O ring on the cap, given that the vertical channel into the tank appears to bypass it?

It isolates the pressure chamber in cartridge from open end in the neck. Exactly how this works when the head bleed to the chamber also connects with the open tank remains intriguing. My suggestion of the hydraulic lock/shock principle is an obvious subject for discussion.

TC

Originally Posted by Jdwoodhou

I think your theory about scale in the header tank is correct. I live in a hard water area so I've used deionised rather than tap water mixed with my OAT antifreeze from new (and flushed the system a couple of times) and have no scale.

Yes, I do the same. Hard water and OAT should be avoided IMO. All antifreeze formulas contain calcium sequestrants to 'mop up' and hardness in the water. I'm sure they'd use deionised on the production line, but it's inevitable that the vast majority of coolant replacements are made from tap water and some areas of the country contain a lot of calcium and magnesium. These are ions that will precipitate as scale when the water is heated, especially if there's no sequestration. But I do suspect the 'organic acids' forming the basis of the OAT inhibitors are responsible for the scale. From my observations, it's completely insoluble in acids and alkalis and can only be removed mechanically. All divalent metal ions (the so-called Group II metals -calcium, magnesium, barium, etc. etc.) come out of solution in the presence of most acids, organic or otherwise.

TC

[Heddy]

What a fascinating post! I've always been at odds to the idea of filling whilst cold. I've done this to find the level is out of sight in a few days. If I check with a hot engine the level is fine, and a pinch of a hose squirts coolant into the neck. It certainly helps to understand the secret life of the coolant cap!

[RobSun]

I recently found that the small bleed hole in my header was blocked and couldn't get any wire to go through to clean it.

I took the bleed hose off held my foot pump hose on the end of the hole and pumped the pump with my other hand and all the muck came flying out.

I looked for the other hole that you show in your pics as I had heard they can block also and couldn't find it. The interesting thing is I looked where it is shown in your picture but at that point there is no hole. Could there be other designs out there or am I not looking in the right place.

[Salad-Dodger]

Here's a few more pics as promised. The over-flow connection on the black tank vents to the outer ring channel under the cap.
I think your theory on the OAT is correct. If you have a look at the pics of the black tank you will see a lot of scale. I've tried hard to disolve this but no luck so far. This tank was one I bought off ebay and is not off one of my engines! I don't use OAT as I believe its not as good as the latest red long life anti-freeze. I'm not up for a discussion on this. Its up to the individual what he/she wishes to use.
I'm going to buy a couple of level sensors from the BMW range as I think that it was MGR's intention to fit a sensor at some stage, especially as they have the same byonet fit as the black tank. I may pay a freind of mine a visit who runs a vehicle dismantlers local to me. With a bit of luck, he may have some BMWs I can have a look at. Steve

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Originally Posted by RobSun

I looked for the other hole that you show in your pics as I had heard they can block also and couldn't find it.

It's not visible by eye from outside the tank no matter how you try. You may well observe it using a dentist's mirror. To get a boroscope into a position where it can see the hole would require pushing it into the tank via the bottom outlet. Not something you would attempt routinely. In my opinion, this particular hole is not worth worrying about.

I personally doubt the primary bleed hole ever blocks up. It's more or less the same bore as the connector nozzle and is always open to coolant flow. In contrast, the small bleed hole in the neck is blocked off and stagnant for most of the time. I suspect small floaters of debris in the bleed coolant may settle in there because it's a dead spot.

TC

Originally Posted by Salad-Dodger

Here's a few more pics as promised. The over-flow connection on the black tank vents to the outer ring channel under the cap.

Thanks again for these. So, the black tank overflow is just a gravity discharge from the annular space. With a standard cap in position, a violent pressure release would send everything out as per the buff tank (through the finger grips onto the tank), plus a some through the overflow pipe.

I don't use OAT as I believe its not as good as the latest red long life anti-freeze. I'm not up for a discussion on this. Its up to the individual what he/she wishes to use.

I agree entirely, but you should be aware that the latest red long life formula is also OAT. But it's no longer a cause for concern in terms of polymer silicone softening. The old OAT culprit was identified and eliminated years ago. So if it's red or orange, it's OAT. If it's purple (like the VW-Audi G12/G13 system) it's also OAT. Only the green and blue types are non-OAT. And there are some types that are combinations/hybrid (HOAT types) that rarely get discussed.

TC

[Salad-Dodger]

Well you learn a little every day! I work for TPS (VW parts arm) in the mornings and they sell all the variants of anti-freeze. Blue, Red, and OAT, plus a couple of others that just sit on the shelf. I can't help wondering why they still stock the OAT in view of what we have discussed! Its quite expensive too. Steve

[Arctic]

Hi Cyrano
Excellent thread most informative and very detailed photo's and graphics to go with them this should help a lot of members to understands how the coolant cap and systems works, always know that if you post that a good read in going to be had. Arctic.

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Originally Posted by Mike Noc

One thing puzzles me - what is the purpose of the lower O ring on the cap, given that the vertical channel into the tank appears to bypass it?

Hi Mike, further thoughts. As I perceive the mechanism, the sole purpose of the o-rings is to isolate the pressure chamber. It wouldn't work as it does (or at least as I propose it) if there was this additional free path through the bypass drilling directly into the tank. Because the bores of the two routes are different, I'm suggesting the diaphragm lifts before the hydraulic pressure simply dissipates via the wider bleed hole. It's the only mechanism I can come up with from what I observe physically. But it does need discussion. As I've said in the write-up, what I'm suggesting is a starting point for discussion that might, hopefully, explain everything.

TC