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??
[demime 1.01d removed an attachment of type text/x-vcard which had a name of
coudesluijs.vcf]
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