I got all all mixed up re which one needs a sprung pressure cap: the header tank or the expansion tank (my 1975 XJ6 is stock and does not have an overflow tank).
I reason that the expansion tank needs a sprung cap as it is a mere “extension” of the header tank, and coolant movement to it shouikld allowed freely but… looking for info on the web is showing everything and its contrary while the terminology is very … variable.
My understanding is the following:
- Header tank: a small chamber at the front of the manifold. Its purpose is to increase the volume of coolant available to the engine and allow the filling up of the system.
- Expansion tank: a ~2 liter tank placed on the inside of the front left fender that serves to accommodate the volume fluctuations of the coolant as the engine temperature goes up and down. It should never be empty to avoid air going into the cooling system. If it is overfilled, when the engine is warm, excess coolant would be pushed out of the system into the outside world (or an overflow tank if fitted).
- Overflow tank: to catch possible coolant overflow and avoid spillage: whatever gets in there, never goes back into the cooling system, it is waste.
Please put me out of my misery, I will take the humiliation