Cooling System

[Mike Wattam <MikeWattam@domain.elided>]

Hi David

It would appear you have suffered from 'after-boil', a not uncommon problem
on Stags.  If stopping suddenly from high speed and high load, the latent
heat in the engine is quite capable of boiling the water off as steam. 
Therefore, if you need to stop suddenly like that it is best to leave your
engine running for a short time until you are confident the temperature has
stabilised.

When your engine is warm it should be developing lots of pressure and if it
is not, you are in danger of the water heating up beyond the atmospheric
pressure boiling point so you could suddenly get your water turn to steam.

As you have not mentioned this I have assumed you have no external water
leaks.  So, you have either air being pushed out of the expansion tank cap
(easily mended with a new cap) or you have water escaping internally.  I
would suspect the expansion tank cap - as it allowed the engine water to
turn to 'steam' in the first place it may have been opening at low pressure
anyway.

The obvious second route for diagnosing this problem - bearing in mind the
work you have had done - is via the inlet manifold rear gaskets which also
carry the waterways to heat the inlet manifold.  It is possible that the
incorrect gaskets are fitted, or that they have been fitted incorrectly -
in which case water from the waterways could get drawn into the engine.  If
thin (TR7 type) gaskets are fitted or the correct gaskets with the drain
slots blocked are fitted to an inlet manifold with warped intake flanges,
this condition could easily occur.  See Glenn Merrell's excellent email
yesterday for info on sorting out the dreaded flanges!

Your local garage probably carries a cooling system tester, which basically
comprises an air pump you put on the expansion tank and it then pressurises
the system to the pressure you need.  In cases like this it is an
invaluable aid to diagnosis

The second possibility is more sinister I'm afraid.  It may be that your
overheat has sparked off a cylinder head gasket failure, or that the seals
on the water pump body have failed, allowing water into the engine.  Before
you strip anything, check the engine oil for water content though.

IMHO - Mike Wattam
Chairman - Triumph Stag Register