I drive a 96 328i. Last week I went into a store and left it running. When I came out the temp was in the red. I let it cool down and drove it home. I changed the thermostat and it was fine for about a week. Last night I was driving and I heard a pop. All of the coolant sprayed over the road. I am about to throw it up on a jack to identify the leak. It seems to be coming from underneath the back of the engine. If anyone has any suggestions what it might be from, it would be a great help. But I noticed that coolant leaked from around the cap and there are traces around some of the hoses on the radiator, like the coolant was pressurized to much. There is also a little black cap floating in the resevoir. Does anyone know what this is, and is there something that could cause the system to pressurize to much? I checked the oil and there are no traces of coolant in it, so hopefully it is not the HG. I have heard that overheating it once can cause it to blow.