I'll never go back to Autohaus Bayern.
I took my M3 in last December for a new radiator. Shortly after the service, the car started severely overheating. I immediately took it back to Autohaus to see what was going on.
I got the car back with a diagnosis of a cracked head - like the last posting, they suggested something like $5,000 in repairs and sent me home with a "do not drive" warning. As an aside, the car was delivered with a check engine light on and significantly low on coolant (took three fills of the overflow tank to fill). Very sloppy work!
I did some of my own diagnosis and convinced myself that the overheating was due to poor circulation of coolant through the radiator. The top radiator hose would get hot when the thermostat opened, but the bottom hose would barely warm up, even when the car began to overheat. I also looked for the classic signs of a cracked head - water in the oil, grimy coolant, or lots of steam, and found nothing.
I started asking questions of Autohaus and was assured that the original radiator failure and the overheating were both explained by the cracked head, which was confirmed by the "absolutely reliable" test for exhaust gas in the coolant.
I had a long conversation with the service advisor about the timing of such a significant failure. While I didn't want to accuse them of blowing my engine during the radiator repair, I simply could not come to grips with the idea that my car, which had never previously experienced overheating, was suddenly and massively overheating after their repair and that the overheating was caused by a blown head on a M3 with only 43,000 miles. At the very least, I wanted to allow for the idea that the overheating had another underlying cause. If we could eliminate it, I felt that I could believe that the cracked head was a pre-existing condition, even though it seemed like an unlikely failure.
The service advisor would not listen to my concerns, and clearly felt that I was just trying to get them to pay for the expensive repairs. He angrily said that he would "throw in a water pump" to see if it would help the overheating problem and offered me the "club discount" on labor for the major repairs.
I decided that I should get a second opinion and took the car to another independent shop. When I described the overheating problem, they immediately asked if I had the thermostat replaced when the radiator was replaced. They said that the BMW thermostats commonly fail in a way that causes them to not open fully, which reduces coolant circulation through the radiator and causes overheating.
I asked about the cracked head diagnosis and was told:
* BMW heads and head gaskets are very robust and almost never fail.
* A head or head gasket failure will essentially always be accompanied by water in the oil, sludge in the coolant, or massive amounts of steam in the exhaust. As noted above, none of these symptoms were present in my car.
* The chemical test for exhaust gas in the coolant is a notoriously unreliable test and should never be used as a diagnosis for a head failure.
I had the mechanic replace the thermostat for around $100 and the overheating was immediately fixed. I have had no other problems with the car since.
I would strongly recommend that you do not take your car to Autohaus Bayern. If you do, and you get a diagnosis of head problems, based on a chemical test on the coolant, RUN to another mechanic for a second opinion.