Just as a possible help to someone out there, not that long ago i replaced the reserve tank, water pump, hoses, fan clutch, belt and assorted other bits of the cooling system (had a new radiator from BMW) due to a failed water pump. I unfortunatly forgot to replace a key componet, the thermostat. Naturally it failed not long after (having been the stock 03 unit) and caused the engine to get hot and blow the vent plug on the reserve tank, this in turn dumped the coolant and left me on the side of the road. I shut it down immediatly. The value add of this note is two part, one, replace the cooling systems on the 540 as a set to avoid this problem. An aftenoons work and not that much compared to a tow and etc. Two, I picked up a new thermostat, vent plug, and BMW coolant, (about $125) and changed it out on the road side in about 15 minutes. Car started and the temp guage went to slightly lower then normal (the car had sat for a little while) and then about 5-10 seconds later jumped to the full hot side. I then blead the air out and tied again. Same funny 2 step and a check engine light. It was my lucky day because it turns out that a great indy BMW guy was at the bottom of the hill and i could roll into his shop. He checked it over and plugged in the computer. He reset the check engine and the engine hot codes and the guage when back to normal. We measured the temp with a probe for about 15 minutes to confirm but car is fine. Prob could have removed the battery to reset but strange behavior to require a computer reset after a themostat replacement.