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
From: Oudesluys <coudesluijs@chello.nl>
Gary,
This is not necessarily correct. If there is little or no air in
the system the pressure will certainly rise to the limit of the rad cap
as the water will expand and as it is non-compressable the pressure will
rise considerably. This is often the case with an airtight rad cap and if
a expansion vessel is fitted.
If there is a lot of air in the radiator, the pressure will not
increase a lot as the air will be slightly compressed by the expanding
water and the increase in temperature.
Kees Oudesluijs
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