why is my BMW 3 Series E90/E91 overheating in stop-and-go traffic?

Solution

1. Preparation

• Gather tools and necessary parts.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the battery to prevent any electrical shorts.

2. Replace Thermostat

• Tools Required: Socket set, torque wrench, coolant catch pan.
• Sub-steps:
  1. Drain the coolant from the radiator into the catch pan.
  2. Remove the bolts securing the thermostat housing and disconnect the coolant hoses.
  3. Take out the old thermostat and clean the mating surfaces on the engine.
  4. Install the new thermostat ensuring it is oriented correctly (check for the "up" mark).
  5. Reattach the thermostat housing and tighten the bolts to the manufacturer's specified torque.
  6. Reconnect the coolant hoses.

3. Repair or Replace Cooling Fans

• Tools Required: Socket set, multimeter.
• Sub-steps:
  1. Remove any necessary components obstructing access to the cooling fan assembly.
  2. Test the fan operation by connecting a multimeter to the fan connector and checking for voltage when the engine is at operating temperature.
  3. If the fan does not operate, replace the fan assembly or repair the wiring as necessary.
  4. Reassemble any components that were removed for access.

4. Flush and Refill Cooling System

• Tools Required: Coolant catch pan, funnel, garden hose.
• Sub-steps:
  1. With the cooling system drained, flush the cooling system with water to remove any debris or old coolant.
  2. Refill the system with the appropriate type of BMW-approved coolant mixture (typically a 50/50 mix of coolant and distilled water).
  3. Bleed the cooling system of air pockets by running the engine with the radiator cap off until the thermostat opens and coolant circulates.