Can you diagnose this?
I have a 2000 323i that just turned over 105k, I took it to my local mechanic two weeks ago to have them change the oil and replace the ball joints. This took a few days for them to complete, in the process they checked all of my fluids and topped them all off. The coolant sensor came on a few days before I dropped it off, and was low. They mentioned that this was one of the low fluids that they replaced. I was grateful for the service that they provided. I picked up the car, the total bill was just under $800. I drove the car home, drove it to church two days later (a total of 30 miles at most round trip) The car then sat for a week, I have been sick and haven't driven much. On Friday I drove it to a class I attend near my church. The coolant sensor came on and the heater wasn't working, so I turned it off. As I pulled into the parking garage the check engine light came on. After the class I had a doctors appointment 10 miles away, the coolant light came on again, then the check engine light. I was 4 blocks away from the doctors office so I decided to just park it there. At the next light my car shut off and took a moment to start again. I drove the few blocks parked it, and it started smoking. I lifted the hood and saw coolant on the block. I let it sit while I went to the doctors appointment. My mechanic was only a few blocks away, I drove it over there after the appointment. The check engine light came on and I watched the engine temperature rise. When I got there we looked under the hood and found the coolant reservoir to have a large crack down the side, also a large crack in one of the pipes leading off of the radiator. My mechanic replaced the reservoir, the broken pipes, and the thermostat. They started it up, it was still overheating and the coolant wasn't circulating properly the top tube was hot while the lower was cool. So they replaced my water pump, now it cools but the engine but the heater doesn't work. They flushed the heater core and now say that the heater works. I am a little irritated now after just having an $800 repair I am now looking at another $900 bill.
Sorry for the long story, I am wondering if you guys can diagnose what you think caused this to occur. Why would the reservoir heat up so much that it would have a 4" crack in it?