Brian came across this problem, not as a result of a cooling issue,
but because the owner's car was not revving freely to 6000rpm.
The problem was down to his pressure cap on the expansion tank!


We all know that water boils at 100 degrees Celsius.
Well that is true at ground level, but if you go to the top of Everest where
the atmospheric pressure is much less, you can successfully
make your cup of tea at only 30 degrees!
Similarly, if you increase pressure, then the boiling point increases.
 This is the job of a radiator cap - to obtain a higher pressure
inside the cooling system, but without running the risk
of blowing it up!


If the pressure in the system was 10psi, then the water
would boil at 100 degrees Celsius.
 The standard Capri expansion tank cap has a pressure rating of 13psi
before it will open and allow excess pressure to vent.


The original standard Capri cap, marked as 13lb pressure


In this case, the owner's problem was that his cap was not holding pressure.
Cap replaced - engine running fine!


If you are sourcing a replacement cap yourself, here is a quick explanation
of the different markings you may come across, as confusingly, several
different units of measurement are used to describe pressure.
Standard atmospheric pressure at sea level is correctly quoted in
kilopascals and is approximately 100 kpa.
This is sometimes converted to other units of measurement such as
1.0 bar or 14.7 psi (pounds force per square inch).
The workshop manual for the 2.8i Capri specifies a cap pressure of 12 - 15.7 psi.
The 13lb standard cap mentioned above converts to 0.9 bar.
Confusingly, that doesn't mean that the cap opens at slightly less than normal
atmospheric pressure (!) rather it is that amount OVER standard
atmospheric pressure.....or in other words, almost double.
Most newer replacement caps have "100" marked on them,
meaning 100 kpa or 1.0 bar or almost 15 psi.
This will be fine for your Tickford and will have the effect of making the water
in the system boil at around 120 degrees Celsius.


A Motorcraft cap marked as 100 kpa


Please don't be tempted to go for any of the racing-style caps as these are
designed to maintain a higher pressure and you don't want
your Tickford running hotter.


Also, do be careful that you order a cap for the EXPANSION or HEADER TANK
and not the RADIATOR by mistake.
The standard 2.8i Capri also has a cap on the radiator, but this doesn't do
anything other than cover the filling hole.
It is not sprung to release at a certain pressure because it needs
to let water run freely between radiator and expansion tank.