John many thanks for being patient re my use of logic creating questions in my mind relative to the specific operation of the cooling system on my mid 80`s +8 , which not unusually would seem to be configured differently from that which is in place in your V6, despite the use of a remote tank in both our Morgan`s
It does indeed seem that the remote/expansion tank can be found in a few different Morgan`s and that as such it would seem that their operational function may differ in terms of what type of cap goes where, perhaps that the necks on both the rad and expansion tank are similar on some Morgans, it could perhaps be a simple matter to refit caps the caps in the wrong location during a rebuild...?
When you type of the vent opening in the pressure cap, apparently there may be two different types of vented cap as per the Jaguar web page I mentioned, one which is opened and venting and the other which is closed until vacuum within the system might suck the valve opened...?
It is my guess that a pressure cap of the type fitted to my +8 may not need to be a two way cap. The primary action being to blow off pressure at or around 15PSI, and a secondary action activated by any possible vacuum building up in the system as the coolant contracts back into the rad.... I suspect as the caps operate on my +8, the pressure created within the tank by the air compressed within it as the airspace having been reduced by expanding coolant, that same pressure will act upon the coolant to assist it to return to the radiator when the engine is shut down and begins cooling ...?
Alternatively, I presume the same dual action pressure cap as is fitted to my +8 could be fitted to a rad with a remote tank utilised as a recovery system, whereby the remote tank would have a vented cap open to atmosphere. Thus allowing any coolant displaced by the rad to expand into the tank above whatever pressure the rad cap is set to pass hot coolant at (likely much less than 15psi) and once things start to cool down the secondary and closed valve within the cap, will be caused to open by vacuum created as the coolant within the engine contracts...
Returning to thinking on my +8`s cap were it only a one way pressure cap and with a blanking cap in position on the rad (as fitted), it would function no differently than it appears to have done during my 20 year ownership....?
As ever open to suggestions of where I have misunderstood the function of MY +8`s system and where a dual functionality of the closed to atmosphere cap (as fitted) might be required...
Just passing the time thinking in type.
