The pressure in the system rises fairly quickly to near cap pressure as the
engine warms up, maybe only five minutes from cold because the coolant
increases in volume by about half a litre from cold to fully hot. Each time
the engine cools down, the cap vent valve allows air back into the radiator to
prevent a vacuum forming so each time the engine warms up this extra air has
to be rejected again and the only way out is via the cap. I spent some time
with data recording equipment observing this and was surprised how quickly cap
pressure is reached, even on a system that runs at 23psi.
The bellows
thermostats are by design sensitive to coolant pressure. A liquid similar to
ether inside the bellows boils at the desired temp which raises the internal
pressure and opens the thermostat. If extra pressure is applied to the outside
of the bellows then the point at which the liquid inside boils rises and
therefore the opening temp rises.
Andy.
--- On Mon, 1/4/13,
warthodson@aol.com <warthodson@aol.com> wrote:
> From: warthodson@aol.com
<warthodson@aol.com>
> Subject: [Healeys] BJ8 radiator cap issue
> To:
healeys@autox.team.net
> Received: Monday, 1 April, 2013, 8:48 AM
> Kees,
>
You are correct, however I am assuming an original Healey
> type cooling
system
> without an expansion vessel & an original style radiator
> cap. I
agree that if
> the radiator is filled with water such that there is very
>
little air in the
> system, then initially the non-compressible water will
>
expand & the pressure
> will rise up to the pressure rating of the cap. At
that
> point the "excess"
> water will be expelled & the available space will
be
> replaced with
> compressible air. From then on, I do not believe the
>
heated water &
> compressible air will ever reach the pressure rating of the
>
cap, unless of
> course the water temperature rises above 212F to the
>
pressure/temp equivalent
> of the specific cap.
>
> David nock's explanation
is based on the assumption that the
> thermostat is a
> bellows type & the
bellows is operating correctly at or
> below 7 PSI & the
> bellows is
collapsing at a pressure somewhere between 7 PSI
> & 10 PSI. This
> does
appear to be a logical assumption/explanation. However,
> I believe the
>
original post indicated that the temperature returned to
> normal after the 10
> PSI cap was replaced with the 7 PSI cap. I would think that
> if the brass
>
bellows had collapsed it would not function correctly at the
> lower pressure.
> Replacing the thermostat with the type David describes is
> the logical next
> step. The only thing I do not like about those thermostats
> is that they
are
> 160 F. Of course, most of our driving is such that the
> thermostat will
be wide
> open & the operating temp will reach equilibrium at some
>
temperature
> independent of the thermostat control temp.
>
> Gary Hodson
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