[Supras] FW: A/C booster fans

[Eric Boeck]

Ahhh ...  MOST important, is that it keeps the coolant under pressure.  A
liquid boils when it's vapour pressure equals that of its surrounding
atmosphere.  Vapour pressure in a liquid is most often increased by heat.
So if you increase the surrounding pressure, you also increase the vapour
pressure neccessary for the liquid to boil, and therefore the liquid can get
hotter without boiling.  Here's a P/T diagram for water, and you can see
that at a pressure of 200 bar, water boils only at 374C.
http://www.pa.msu.edu/~bauer/talks/Honnef2000/sld003.htm
 
Also, by combining antifreeze and water, the boiling point for that solution
goes above the 100C for water alone at standard pressure (1 bar), and it has
to get MORE energy (heat) into it to get that vapour pressure equal to the
surrounding pressure, and therefore boil. (check the container of antifreeze
you used for the boiling point increase for the combination you used).
 
Here's a link that might explain it better:
http://en.wikipedia.org/wiki/Phase_diagram
 
Long story short, under pressure, the antifreeze can get hotter without
boiling.  Rad caps are designed to contain a certain pressure, but if the
pressure gets too high, the cap releases, and THAT liquid / gas flows into
the overflow container.
 
As for what can break, well, I guess the spring can go soft, and that will
release the pressure at a lower value, and therefore, the boiling point of
the rad fluid is lower.  Any mechanic *should* have a rad cap pressure
tester.  Just get him to test the cap.
 
As you see in the pic I sent, the one on the far left has too small a rubber
area to seal the big hole in the place on the rad where it goes (far right).
The cap in the middle WILL and does seal the opening.  The cap on the far
left is the one I got when I went to Toyota and just asked for a rad cap for
my 91T.  The one in the middle is the brand new one, and I asked for the
specific PART number, and NOT the cap that goes with my *car*.  The two are
as different as night and day, and if you compare your rad cap to the pic I
sent, and compare the opening in the rad to the one in the pic, you should
be able to see if *you* have the correct cap.  The other way to tell is when
the car is cold, just squeeze the upper rad hose.  If you hear bubbling in
the overflow, your cap is either bad, or the wrong one.
 
My car boiled over a few minutes after I turned it off because of heat soak.
(no moving air, the heat builds up under the hood).  Since the rad cap
wasn't keeping the fluid under pressure, it boiled at that lower
temperature.  Now that I have the correct cap, it keeps the rad under
pressure, and the heat soak temperature no longer is enough to cause
boilage.
 
HTH's
 
Eric
91T
 
PS.  Sorry for being long winded, but that's the chemist in me coming out.
:)
 
 
  _____  

From: Charismatisch at aol.com [mailto:Charismatisch at aol.com] 
Sent: Thursday, September 14, 2006 2:56 AM
To: eboeck at cogeco.ca
Subject: Re: FW: [Supras] A/C booster fans


Hallo Eric,
 
not being a professional mechanic, one thing puzzles me about your comments
on the rad.cap. I always thought that a rad. cap was only there for 2
reasons:
 
1. To get access to put more water/coolant in
2. To stop water/coolant spilling out of the rad. when the car is moving
 
What can be broken on the original rad. cap or wrong with the replacement
rad.cap on a Supra, and what is its real function? How come the car can boil
over if it doesn't function properly, and how is it supposed to work/what
does it do?
 
Many thanks,
Derek