Ok, so I finally took my 2010 128i to the dealer because I got sick of having not heat. I was fully prepared to buy a new thermostat or maybe even a heater core. However, here's what the dealer found. With the coolant system under pressure, they found the heater core was clogged. Ok, I can see that. Then they found that the pressure test kept losing pressure. Ultimately, they found pin hole (porosity) leaks on the intake side between cyl #1 and #2 on the block below the intake. WTF!!! I bough this car used with 56,000miles in July. I bet the previous owner used some sort of stop leak chemical to fix the pin holes and in the process clogged the heater core. Heater core replacement sucks but I know it will be clog free and good for quite a while. That's not my worry. Block porosity has me thinking the repair won't last. The dealer fix to the pin holes are to clean and prep the block area and apply a type of epoxy. This would be to the outside wall as there is no access to the inside to complete the repair. Anyone heard of this? I had a 1991 318is with the M42 motor and it had a leak at the head gasket where green coolant ate away the aluminum. But they filled with liquid metal and machined. Not sure about an epoxy repair on the outside of an engine block.