some partial success......
carried some more investigation,
running the engine at fast idle it transpired that the radiator cap was lifting just before the temperature needle reached the red - ditching coolant down the overflow tube - in turned dropping the pressure / level in the cooling system causing it to boil.
I replaced the rad cap with a new 15 PSI rated one and ran at fast idle (with a lot of thrashing) to get the temperature into the red without the cap lifting - Hooray I thought
I then went for a drive, things seemed perfect, temp sat just over half way
then I got to the dreaded bypass, the moment I got over 40 mph the temp gauge shot into the red and the new 15psi cap lifted, luckily I had brought more water with me (it only needed 2 litres to top up)
I did however notice a small leak on the heating matrix. It could be that there is a sufficient leak here to drop the water level and system pressure to a sufficient level that the water pump becomes ineffective and the coolant boils.
So I have bypassed the heating matrix - but ran out of time to test it.
Failing that I have been reading up, as the large temp rise only occurs when the van is flat out I am wondering if I am getting exhaust gas leaking into the cooling system. Next step is to test that / get the head back off to inspect it.
Its mad that it runs fine at 35 but goes potty at 40
The Dog is home....
http:www.singleusemedical.co.uk
my new business