[Healeys] Closed system- little confused

Oudesluys coudesluijs at chello.nl
Thu Jul 29 15:37:19 MDT 2010


Jeff,  this question has come up with one of the other listers too. My 
answer was:


When the engine cools down the fluid and trapped air in the engine and 
radiator will contract causing a vacuum.
The radiator cap has two valves, one that opens when the pressure 
reaches a certain maximum of generally 6, 9, 10 or 12psi, as is written 
on the cap, when the engine is heating up, this is the large plate with 
a biggish spring underneath and a rubber seal on top (this seals against 
the bottom of the filler neck, below the overflow pipe), and a second 
smaller valve that opens when there is a vacuum when the engine cools 
down, this is the small plate in the center of the pressure valve plate 
on top of the rubber seal. If you take of your radiator cap you can lift 
this small plate from the rubber seal with your fingers with very little 
effort.
The vacuum will suck the coolant from the overflow  bottle back into the 
radiator, providing the hose is connected to the bottom of the bottle or 
when the hose is inserted through the top is extended to nearly reach 
the bottom so that fluid instead of air is sucked up.

After this process has been repeated several times and if the overflow 
bottle has been kept 1/2 full when cold, most of the air will have been 
expelled from the engine and radiator, causing a radiator that is full 
to the brim, providing you have a thin but fairly stiff rubber seal 
between radiator cap and radiator filler neck to make a really airtight 
seal. This is particularly important if you have fitted a thermoswitch 
for an electric fan in the header tank or top radiator hose.

This thin rubber seal is usually not there. The manufacturers relied on 
a flexible brass seal to prevent unpressurized coolant passing ( however 
this is not an air tight solution and will not hold a pressure or 
vacuum) and sticked with the radiator being filled only to about1/2" 
over the core and regarded the header tank more as an expansion vessel 
like in the days that there was no overflow bottle and a boiling engine 
would spill its contents on the road.

I hope this helps.

Cheers,
Kees Oudesluijs
NL


Jeff Pelletier wrote:
>  Ok... im going with the closed radiator system you guys have talked
> about in earlier post..  but im a bit confused as to how the
> water/antifreeze gets BACK into the radiator..  We put a gasket ontop of
> the radiator just below the cap to seal things off--correct.. ...  then
> you have the inlet/outlet hose/pipe.. then at the bottom of the neck you
> have another gasket with the pressure of the  spring on it...  as the
> car cools how is the water going to get past the gasket at the bottom of
> the neck back into the radiator??

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