That sort of makes sense, because until the thermostat opens, there will be no water circulating through the heat exchanger.
Originally Posted by vyv_cox
There are two difficulties I have with this though, what Vyv alluded to before regarding the pressure differential, there is no area within the heat exchanger that is only separated by any kind on seal or gasket, its all brazed up, it would have to be a ruptured copper pipe. Second point the problem seems to be worse when the engine has been run hard for a long period of time.
But I thank you all, I do have other areas to investigate now, but dont stop there, keep coming up with ideas!
The only way to be sure is to but a pressure test on various parts. Heat exchanger, calorifier. The generally accepted way is to positively isolate the unit under test (using plugs not valves which may leak slightly) apply test pressure with test pump, isolate test pump and monitor pressure drop over 24hrs. For best results monitor temperature as well and apply a correction. Don't apply to high a test pressure, refer to allowable pressure of component under test. Usually test to 1.5 times working pressure.
This will reveal where the problem lies. Test pumps available from HSS to hire or Screwfix to buy.