The radiator overflow tank serves as the reservoir for engine coolant that is heated to its boiling point and would otherwise rise up and out of the radiator. Working in conjunction with a thermally activated spring built into the cap, the overflow tank creates a closed cooling system that is much more reliable and practical than previous designs. As the temperature and pressure rise to the coolant's boiling point under pressure, the cap spring activates and allows the heated liquid to flow up and into the overflow tank. The fact that the system is under pressure means that the liquid reaches a higher temperature before boiling, further increasing the effectiveness of the system. If you had a low coolant warning light , the expansion tank may be one of the parts of the cooling system to blame. If you work on any cooling parts, including the expansion tank, you must bleed the system/purge. Bleeding the cooling system involves removing trapped air that can clogg the radiator, water pump and so on. Trapped air can cause hoses to burst and also fail the expansion tank or blow head gasket . Reasons that a low coolant warning needs immediate attention. Work notes: Trapped air will have heater core blow cool air when temp is set at high. If you notice this repeat steps. Wait for engine to cool and "bleed the system" 1. fan on low heat on high 2. Add coolant to indicated marks never over fill 3. you can leave cap off or bleeder off to watch for bubbles purging 4 place cap back on start the vehicle 5 if you still have bleeder on ...release it and let the air out( you may lose a little coolant)////if you released bleeder before starting , wait for bubbles to go away...you may still lose a little coolant doing it this way....both ways work the goal is to remove trapped air out the system. There is also an electronic way to bleed the system activating the electronic water pump so it can run itself to purge the cooling system of trapped air. This will allow you to bleed the system with out spilling any coolant. A battery charge would also help. The reason for the battery charger is to prevent the battery from draining while the water pump is activated. there would be no need to start vehicle with this method. ...I may do a video on this in the future. 6. Good idea to watch coolant temp in hidden service menu but not really required when the goal is to remove trapped air. You will know when your vehicle reached normal operating temp when fan is on. Hidden service menu ...coolant temp access: https://youtu.be/8nRC4mTo1A0 7. Drive vehicle and make sure you heater works and you should be all set. If its blowing cold air then air is trapped in cooling system. At that point bleed system again. 8. Good idea to bleed system on an incline rather flat surface ....30degree jack stands or a slope really does help, but not required. Use Bmw coolant with distilled water 50/50 mix Good idea to use a coolant funnel even tho I did not in this video...this will prevent overspray of coolant. Thank you for watching #bimmajoc #bimmergurus #Raoul #Jacques
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