Overheating in Traffic, Fine on Highway?

This is a classic BMW symptom with a very specific set of causes. If your temperature is fine at speed but climbs in stop-and-go traffic, here's what's happening.

Why It Happens

At highway speed, air flows through the radiator naturally (ram air). In traffic, there's no airflow — the electric fan is the only thing cooling the radiator. If the fan isn't working properly, the car overheats.

Cause #1: Electric Fan Not Working (Most Common)

The radiator fan motor has failed or isn't running at full speed.

Quick test: Turn on the A/C and look at the fan. It should spin. If it doesn't — that's your problem.

Causes:

• Fan motor burned out
• Fan resistor/controller failed (fan only works at one speed or not at all)
• Blown fuse
• Wiring issue

Fix: Fan assembly replacement — €200-500

Cause #2: Fan Running but Not Enough

The fan spins but at reduced speed due to a failing controller or motor.

Test: With A/C on and engine warm, the fan should be blowing hard enough to feel significant airflow in front of the car.

Cause #3: Partially Clogged Radiator

The radiator is partially blocked internally (scale, debris) or externally (bugs, leaves, dirt between the radiator and A/C condenser).

Test: Feel the radiator surface — cold spots indicate internal blockage.

Fix: Clean the exterior with compressed air. If internally clogged, replace the radiator.

Cause #4: Thermostat Partially Stuck

The thermostat opens partially but not fully, restricting coolant flow.

Test: Feel the upper radiator hose when the engine is at operating temperature. It should be hot. If it's lukewarm, the thermostat isn't opening fully.

Fix: Replace thermostat — €80-200

Cause #5: Weak Water Pump

The electric water pump is running but at reduced capacity (impeller partially separated or motor weakening).

Test: Difficult without diagnostic tools. If the pump is over 80,000 km, replace it proactively.

The Fix Priority

1. Check the fan (free — just look at it)
2. Check the fan fuse and relay (free)
3. Check for radiator blockage (visual inspection)
4. Check thermostat operation (feel the hoses)
5. Consider water pump if everything else checks out