Ok, theoretically there should be no pressure within the coolant system once it's been left for a while but realistically you will find a small amount of pressure in the system when you release the cap even if it's been left over night. There's been a car with a BXE engine in our workshop all weekend for a service this morning, I removed the cap and a small hiss if pressure was released, the customer has not complained of any faults or coolant loss it's just normal behaviour of these systems. If there was a large amount of pressure after a short period of running then that is not normal.
Block leak tests often prove unfruitful when checking for coolant to exhaust gas contamination on diesels unless coolant is definitely being forced out if the cap by exhaust pressure. It's usually escaping through the exhaust ports and being burned of or into the bore and being burned with the fuel but there would be visable steam from the exhaust unless the leak is minute.
Also it's very rare to see coolant to oil contamination, it's the other way round where oil appears in the coolant. It occurs this way as the oil pressure is higher than that of the coolant system.
