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BMW 3-Series and 4-Series Forum (F30 / F32) | F30POST > Technical Forums > Mechanical Maintenance and TSBs: Break-in / Oil & Fluids / Servicing / TSBs and Service Bulletin > Why the need for a coolant bleed procedure?
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      03-15-2019, 09:02 AM   #1
applehusky
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Why the need for a coolant bleed procedure?

After the radiator vent hose on my '14 335i gt broke and let some coolant out, I had to follow a procedure for bleeding the system after filling the coolant back up.

I was wondering if anyone knew why they don't tell you to just turn the engine on and drive around in order to get the air out.

I have thoughts but I'm wondering if anyone here "in the know" could shed some light on this.
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      03-15-2019, 10:42 AM   #2
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If you don't bleed the air out you can't fully fill the system with coolant. There will be pockets of air trapped in the hoses. If you run the engine it and the transmission could overheat and cause damage to a number of components, starting with the water pump and getting progressively worse from there.
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      03-15-2019, 10:46 AM   #3
applehusky
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Interesting
I knew why the bleeding needed to happen, just thought it was odd that it had to be done separately from starting the engine.

So it's basically just to minimize the risk of the engine overheating while filling the coolant?
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      03-15-2019, 12:43 PM   #4
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No, it's to get rid of air in the system. The system is sealed, so any air trapped in it will stay in it unless purged. That's the reason why the coolant must be put into the circuit under pressure as well. It's not your father's Oldsmobile.
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      03-15-2019, 12:55 PM   #5
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Quote:
Originally Posted by Billfitz View Post
No, it's to get rid of air in the system. The system is sealed, so any air trapped in it will stay in it unless purged. That's the reason why the coolant must be put into the circuit under pressure as well. It's not your father's Oldsmobile.
oh I knew what bleeding was and why it needs to happen. sorry for not being clear.

my question had to do with the way BMW chose to have it done. I'm having a conversation with a friend of mine about how you bleed the system on a BMW as opposed to another type of car. in his opinion, BMW's procedure is unnecessary complicated. however, I think there's a good reason behind it because procedures like these don't exist just because.

so, this is my question: what is the design problem that BMW were trying to solve by creating a procedure that bleeds the coolant circuit without the engine running?
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      03-15-2019, 02:45 PM   #6
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I don't see a design problem per se. Coolant doesn't fully circulate through the system until the engine comes up to operating temperature, so if you wait for that to happen before the system can purge trapped air damage can occur before the air gets out of the system. If you purge the air without running the engine that can't happen.
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      03-17-2019, 07:47 AM   #7
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Quote:
Originally Posted by Billfitz View Post
I don't see a design problem per se. Coolant doesn't fully circulate through the system until the engine comes up to operating temperature, so if you wait for that to happen before the system can purge trapped air damage can occur before the air gets out of the system. If you purge the air without running the engine that can't happen.
that makes sense
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      03-18-2019, 12:16 AM   #8
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What he's referring too is a thermostat, the coolant isn't circulating until it's up to temperature, thermostat opens and then it goes through the whole system.

Don't forget to turn your heater on fully so the water can pass through the heater core as well.


Also, no car can be bled by driving it around, even an oldsmobile or a honda, they either have a bleeder screw at the highest point of the cooling system, or it's done through the overflow/radiator cap.
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