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Old 12-08-2015, 10:48 AM
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Default Question on coolant flow

I have a leak in my E36 coolant. I found it last night and fortunately a simple repair. But I don't really understand how the coolant flows into the radiator with cars that do not have a radiator fill location. The same applies to the E30, I just never took the time to understand how the fluid made its way to the radiator.

The E36 expansion tank has a hose at the base which is connected to some location on the head. as the tank is higher the fluid obviously continues to fill the voids in the block until it is filled. Then I would think the once the block is filled it would somehow make its way into the radiator. But I cant seem to figure out how that is happening. The only hoses which go to the radiator are from the thermostat, which I have always presumed was closed until the temperature required it to open allowing cooler fluid to flow from the radiator.

So I am missing something here and sure it is simple. But if anyone can explain I would appreciate it.
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Old 12-08-2015, 11:11 AM
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It doesn't happen very well on BMWs

*IF* the system has no air bubbles you can view it as a big closed loop. There is a pump somewhere in the loop. As it does it's job coolant goes round and round. A thermostat can partially block the loop to slow down the rate.

Problem comes when there is an air bubble that reaches the pump. It can't pump air. The simple solution to this is put the pump at the bottom where it can be gravity fed and the fill tank at the very top to bubbles purge themselves. This seems no longer possible on many modern cars due to packaging constraints. So you end up w/ bleeder screws (I know the E30 has one, can't remember about our E36), etc as ways to purge the air from the system. Once the system is purged (and stays that way), you can plumb the expansion/fill tank anywhere in the system and it doesn't even need to be the high point! Think of a typ office water cooler w/ the big upside down jug on top. There is no seal keeping it from making a mess. Just a small amount of water covers the neck in a pan down in the cooler. The big jug is the 'sealed' engine system. The small reservoir of water is like an expansion tank BELOW the level of the main system. Physics can be funny sometimes.

The coolant path in something like a Porsche 911 has gotten so long and complex there is no practical way to put bleeders at all the high points. So you eventually say, F-it!, get rid of all of them and 'simply' require vacuum fill.
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Old 12-08-2015, 11:29 AM
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Steve, thanks...when the car is running and the pump is turning and the thermostat opening and closing, I suppose I can understand how the fluid is flowing. But when the car is cold and I am filling the expansion tank to get fluid into the system, if the thermostat is supposedly closed, how is the radiator getting any coolant into it. the only two main hoses connected to the radiator are coming from the thermostat housing...and of course the small hose to the top of the radiator...does it leak by the thermostat or someplace in the thermostat housing?

E36 has a bleed valve on the top of the expansion tank...plastic piece which also breaks.
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Old 12-08-2015, 11:53 AM
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Tstats typically have a small hole in them. So a LITTLE flow (or air!) will go through even when cold. I remembered both the E30 & 36 being a bit of a pain to get bled. Ahh, the good 'ol days
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Old 12-08-2015, 12:06 PM
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have always bled the e30 by raising the front of the car to ensure the bleed valve was the highest point in the system. has always seemed to work...lets see if the e36 does the same. Thanks for input...if the hole is small I guess it is enough to allow the radiator to fill...
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Old 12-10-2015, 11:28 AM
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Tito:

I do the same way with E30 (and with E39). Front of car high and bleeder screw removed. Fill expansion tank and fill upper heater hose through small hole where bleeder screw removed.

Don't know about E36 but E39 (most) have an auxiliary in line electric coolant circulating pump (driver side, lower right corner, behind radiator) that will activate with heater set on "resting heat", engine not running.

"resting heat" is designed to pull latent heat from the system and keep peeps sitting in non-running car warm. Side benefit is can use Aux coolant pump to fill/circulate coolant with system open, heat set on full hot and engine off.

Typically always have to top off after a post-refill heat-cycled drive.

Good luck.
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Old 12-16-2015, 03:17 PM
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After a few days of research on bmw e36 coolant system flow and issues, I have discovered the thermostat in the e36 when open allows coolant into the engine (directs it to the water pump) from the lower hose vice opening to allow it out of the engine and into the upper hose as a coolant system does on many other cars. Hence with the thermostat closed on a cool engine there is a path to the radiator for coolant when filling.

Lots of other issues I guess with the cooling system, but at least it makes sense how the fluid can make its way around and fill up the radiator with the thermostat closed.
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