The car is 2001 BMW 330i. I have changed ignition coil, spark and IAC. Last week, my car experienced rough idle. Then I changed the coolant temp sensor today. After adding coolant and bleeding, the car ran smoothly in the day time (about 2 hours) without rough idle. I drove the car out tonight. The low coolant light turned on. After driving the car for five minutes with coolant light on, rough idle came back. After restarting the engine, the car became normal again. Any clue for fixing this notorious rough idle issue for E46?
Additional issue: The heater blows only cold air when the car is idle. (I live in Los Angeles. Never use heater.) This might be a sign of bad thermostat. Will this cause rough idle? In theory, bad thermostat might prevent coolant circulating and cause overheating of the engine.