I’m a bit late to this thread but going back to the first picture surely there shouldn’t be a pressure cap on the engine. The pressure cap should be on the expansion tank, that way the coolant expands with heat and flows up to the tank, when the engine cools down the fluid can flow back into the engine. If you have a pressure cap between the engine and the expansion tank once the engine is switched off the pressure cap will close before all the fluid has flowed back into the engine ……at least, that’s the way it works on my car.
