Pressure cap question

[ukbiggles]

Hi

This is a follow up to a thread that I posted a year or so ago about coolant not being fully returned to the radiator from the standard expansion bottle.

The setup is a standard Mk2 approach though the Expansion tank has a 13lb pressure cap fitted rather than 20lb. It ran for 6-7 years just fine with this size of cap.

Problem is that I had some body work done on the front valance which necessitated the removal of the radiator etc in order to get at the valance. As far as I am to judge/remember, the coolant return problem seemed to start then. I had many helpful suggestions from this forum suggesting air leaks in hoses, faulty pressure cap, faulty expansion bottle, faulty return pipe on radiator, early HGF.

I have replaced all the hoses and also renewed the water pump, thermostat and inlet manifold gaskets and even tried a 20lb cap. I still have the same problem. I'm inclined not to think that it can be HGF as the cooling system pressurises OK and doesn't overflow the expansion tank.

One thing was slightly odd (to me) is that when I bought a new 13lb pressure cap, it came with a large loose fitting rubber ring to act as the high pressure seal. This was quite different to the cap that was on the car as the high pressure seal was a metal ring. I attached a photo of the old cap and the new one side by side. I have tried using the rubber ring on both old and new caps and simply cannot fit the cap to the bottle as the rubber ring is too thick.

So finally, my question is this.

Is there supposed to be a rubber ring acting as a seal at the 'top' of the cap. Or is the metal to metal seal normal and sufficient? And as a follow up, if the high pressure is not good, could it result in my coolant loss issue?


Thanks in advance for any further guidance.


Regards

Gordon





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[mjheathcote]

Don't think it makes any difference as the smaller top seal in the picture is the important one.
When the pressure exceeds the rated value, air is released as the plunger lifts, the escaping air then escapes down the small quarter inch or so rubber pipe to the safe area, usually under the car. It the big seal is missing all it means is some of the escaping air is released around the cap area too.
The seal is however very important if the radiator cap is fitted directly to the radiator, and the rubber tube goes to an atmospheric expansion bottle. If the seal is missing or leaks in this application water will not be drawn back up due to the vacuum leak.
As per the header tank thread, I changed my original expansion bottle for a high level header tank over 30 years ago. At the time my original bottle had failed around the brass to plastic bottle crimp, so when the water cooled it was not drawn back up into the radiator, but pulled in air instead.
I would check this closely and 30 years on I would be suprised if any original bottle was water/air tight around this area.

[ukbiggles]

Hi Mike (I think!)

Thanks for your helpful and explanatory reply.

Yes, I had thought that the plastic connector on the bottle was an area of weakness as it could easily become compressed or deformed. On mine, there is a small diameter copper pipe lining the plastic connection so I'd have thought this would increase the likelihood of an airtight seal.

Ah well, need to get my thinking cap back on and do some more experimentation to see if I can narrow down where the air leak is. Perhaps I'll have to go down the header tank route after that.

Cheers

Gord