After service, my car started overheating

Thursday October 18 2018


By Paul Kaganzi

Hello Paul, a few days ago I took my BMW 3 series E46 to a service bay for coolant service. Shortly after the service, the car started overheating as soon as I started driving. It is now parked but we cannot figure out what the young man at the service bay did to it. There are no apparent damaged components and the car was fine before the service. Can you advise? Ramadhan.

Hello Ramadhan, your BMW E46 engine cooling system may have a vacuum lock or simply put, trapped air. This happens when you drain coolant during service and it is replaced by air in the coolant lines or pipes, just like the plumbing in your house. During and after refilling with coolant, the technician must follow a specific routine to ensure that the cooling system lines are vented or relieved of the vacuum, what we also call bleeding.
After draining the old coolant, select an appropriate coolant fluid with a good blend of ethylene glycol with corrosion and heat dispersant additives. I use Shell Coolguard which comes as a concentrate to be mixed at a 50:50 ratio with distilled water. Your cooling system holds about 8.5 litres of coolant so you will need approximately four litres of concentrate coolant. Ask your technician to follow this procedure to refill and bleed the cooling system. Loosen the coolant expansion tank bleeder screw at the top of the radiator and turn the car ignition on without starting the engine. Adjust the temperature settings for vehicle climate system to full warm and blower fan speed to low.
This will allow hot coolant to flow around the ventilation lines and in so doing evacuate trapped air which tends to cause a vacuum lock and overheating. Slowly fill the expansion tank until coolant starts to flow out of the expansion tank bleeder screw. This flowing coolant should have bubbles. Once the flowing coolant is free of bubbles, tighten the expansion tank bleeder screw.
Start and run the engine keeping it idling until it attains normal operating temperature. You can thereafter reset the climatic temperature settings to cool after evacuating air bubbles in the ventilation coolant lines. The BMW E46 engine should run normally without overheating, after that.