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BMW 5-Series (G30) Forum 2017+ BMW 5 Series (G30) General Discussions Why are there two coolant expansion tanks?

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      08-27-2021, 12:20 PM   #1
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Why are there two coolant expansion tanks?

One is labeled high temp and the other is low temp circuit. Anyone know what each of them do? One is in the front of the engine and the other is towards the front on the right side of the bay. I need to add coolant to the one near the fender but the smaller on is full. Seems like the car uses a standard green fluid rather than some being purple or blue.

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      08-27-2021, 02:02 PM   #2
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'High' is for the engine cooling system, 'low' is for the charge air (water cooled intercooler) system.

It's your engine coolant system that requires topping off.
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      08-27-2021, 04:40 PM   #3
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Not the "low" center mounted one does not have a sensor to warn you when low. I was advised by an independent BMW specialist that if low, it can cause the intercooler temps to rise and it goes into a sort of "limp home" mode and explained why a customer was noticing very low performance on their car...He recommended checking periodically. Just don't ignore it. In their customer's case, he had a leak in the system...
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      08-28-2021, 02:23 AM   #4
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I had to add coolant last weekend and was also was confused what the difference was between the two tanks. I'm pretty sure I over filled the center one. I find BMWs floating measurement thing incredibly hard to use. The one on the driver fender needed more and I'm guessing the center one didn't need any. I didn't over fill the fender one because I ran out and didn't feel like mixing more BMW purple.
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