Technical Car Experts Answers everything you need: BMW 525i that overheated with a hose from the block to the thermostat ?

To answer your 1st question yes the radiator cap should relieve pressure prior to anything blowing apart. The cap would have to be tested to make sure it is relieving at the correct pressure. But it would also depend on the condition of the hose, if the hose was weak and it blew before the pressure was enough for the cap to relieve pressure. I have seen this many times myself.

To answer your 2nd question once again it would depend on the condition of the hose of course, but a clog in the system will do it also and depending on the clog it is hard to say if the pressure would equalize. From my experience with this vehicle from when it was new and from owning my own of the same year and model. These radiators do clog easy and the restriction of flow will cause the overheat situation like you are having. Once you took the thermostat out of the picture and the lower hose to the radiator got hard, that says it all right there. With that thermostat out that hose should never get hard unless there is blockage. When you start it cold before putting the radiator cap on look inside the top of the radiator with the vehicle running at 2000 rpm's and see if you have flow. Also on this vehicle once it overheats it almost always causes the heater core to leak, so something you might want to also checkout after you get the flow problem solved. You have already replaced the water pump, thermostat, hoses. And head gaskets on this model are not common for leaking internally and allowing combustion gases into the coolant, but we can't forget about it either. But everything is pointing towards the radiator being clogged.

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Once I replace this exploded hose, how do I flush the radiator and or test it to determine the volume of water that is required to flow through it? I already suspect the clog in the radiator, but I am not sure how to properly test and clear it.
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Depending on the age and how bad it is clogged it might just come down to replacing the radiator. If you already used a flush kit I would say you need to just replace the radiator at this point. The only way you will be able to try and flush it is if is not overheating, but it is so I think you have reached the end of the line with trying to save it.

But before you install a new radiator you are going to remove the engine block drain plug (located on the passenger side of the engine block below the exhaust manifold towards the rear of the engine) remove the thermostat and run a hose in one on the radiator hoses and flush out what ever you can as best as possible. Then once you have the new radiator installed you can use 1 of the Prestone flush kits for like $ 3.00 and flush the system according to the directions on the container.

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