Here is some info on the different caps

http://www.tridon.com.au/databank/do...iator-Caps.pdf

Jose, the article on British Automotive you referenced is slightly different, there are 2 types, one is an expansion tank as per British automotive article, this system does not use a pressure cap for the radiator, but a blanking/sealing cap, so the actual pressure cap is on the expansion tank, this simply adds more coolant capacity as such and as the coolant expands it flows into the expansion tank increasing the pressure etc. when it cools down it has a free passage back to the radiator

The other type we have been discussing is a recovery system, the coolant exits to the overflow tank as and when the pressure cap on the rad opens and then should flow back to the radiator via the second valve on the radiator cap.

A 2 valve cap can be used on a classic system but a classic cap cannot work on a recovery system according to the info in the link above. I can only think that the cap has to be actually leaking / not sealing properly for a recovery system to work without a properly designed cap ?

 

Jon,
I have no clue what's going on haha! All I know is that it works in my 3.8 S type, with a Volvo recovery-expansion tank. The Volvo tank does have a blanking cap, the radiator has a spring cap.

 

Jose, if it ain't broke don't fix it , I will chalk up another mystery unsolved for me.

 

I would have thought after all the discussion here that it would have been solved ???

 

That is what I thought- I was surprised this kept going. Only fill the radiator to the top of the coils, and use a decent radiator cap...

 

Sorry Jose,
The reason it wasn't solved for me is that no one could answer why it works for you, in theory it shouldn't, but it does, I just don't know why.

CSBush is spot on with what he says re standard radiator and cap.

 

As I'm considering adding a hidden & non invasive expansion tank/surge tank to my car. Found this thread & some confusion still seems to exist.

While certain terminology might not suit some, it's all in the cap. See this brief video.


Remember a header tank can be free standing or just the top tank of your radiator.

 

Also remember that from 420 onward Jaguar was forced to go to Header tanks because they moved to cross flow radiators.

 

At last someone that knows that the cap is the the key to this.

 

No problem TJ ~ I've been dealing with standard fitment of surge tanks on cars since my first Alfa at age 18.

 

I just run the hose into a water bottle or coke bottle to catch the overflow.

 

That's fine but it will only work as a surge tank if you have a return valve in the radiator cap.

 

If you don't have a return valve in the radiator cap, that partial vacuum that is created (as the engine cools & provides the suction required to draw coolant from the expansion tank back into the cooling system) sucks the cap down on it's seat effectively increasing it's value by a few pounds until the vacuum is dissipated by leakage & lost or the engine is reheated by running.

 

I've fitted expansion tanks to a few cars in the past (Rovers, Triumphs, etc). A blanking cap, i.e. no spring just a straight seal on the rad, replaces the original pressure cap, and the radiator overflow is piped to the bottom inlet on the expansion tank. A radiator pressure cap is fitted to the expansion tank. The radiator can be filled close to the top, and the expansion tank should be 1/4 to 1/3 full when cold.

They've always worked very well for me.

 

that is the trick, a blanking Cap at the radiator, and a pressure cap at the recovery bottle or tank. That is the reason the XJ-6 radiator does not even have a cap, the cap is at the expansion / filler tank.

 

That is by definition a remote header tank where the whole system is kept pressurised & more typical of a move toward modern thinking. One should note that these systems generally have no excess capacity like a surge tank system has & are mostly used with cross flow radiators with tiny side tanks.

We were really discussing expansion/surge tanks with drawback via a return valve radiator cap.

To increase the capacity of a remote header tank system you can fit a surge tank to that pressure cap outlet.

These old Jags run very low system pressure ~ generally 4 or 7lbs. The hoses, radiator core, heater matrix & waterpump seal can't handle any more than that.

Modern systems are now totally captive & higher pressure.

 

Glyn is correct, don't confuse the two, they are different systems, whilst both appear similar, they work quite differently.