M30 Guys: Coolant expansion tank details needed

[LowR3V'in]

i got one of those too.

i was thinking of even making like a tube with thin abs that goes around the fan to the grill so it gets air from the outside.
might help maybe not but in traffic my shit overheats in the summer.

[CorvallisBMW]

I am using the stock b34 tank that attaches to the firewall, I also lowered the radiator more since it is an e36 M3 rad and has a shorter profile than the e34. I crank the heat and park with my car flat or slightly nose down to nudge air towards the higher tank. Crack the housing bleeder and that's all it ever takes for me. My issue right now is needing another fan for really hot days when the 16" spal won't cut it.

I plan to redo everything when I drop the engine out for a refresh while it's getting boost added. I can say it's at a shop now since not many look here in the m30 section.

How do you have your 16" pusher mounted? Is it directly on the radiator or spaced out a bit? I found that mine was spaced out around 1/2" and a lot of the air was just hitting the radiator and bouncing off instead of going through. Spal makes fan gaskets though, installing one of those made a HUGE difference.

[F34R]

I am using the stock b34 tank that attaches to the firewall, I also lowered the radiator more since it is an e36 M3 rad and has a shorter profile than the e34. I crank the heat and park with my car flat or slightly nose down to nudge air towards the higher tank. Crack the housing bleeder and that's all it ever takes for me. My issue right now is needing another fan for really hot days when the 16" spal won't cut it.

I plan to redo everything when I drop the engine out for a refresh while it's getting boost added. I can say it's at a shop now since not many look here in the m30 section.

[LJ851]

I got my fingers crossed for you as well.

[CorvallisBMW]

Got a new tank cap, replaced the vent line with all new hose (just in case) and managed to raise the tank by fabbing some basic sheetmetal brackets. I can now fill the tank to the cold fill level with the bleeder screw cracked and fluid just barely starts to dribble out. So that's amazing, because now it means my fill level is above the highest point of the engine. Fingers crossed...

[CorvallisBMW]

I don't have a late model expansion tank anymore, but that weep hole is present on quite a few other BMW models- it's how they "self bleed".

that's where the air from the radiator comes out.

Ya, but there's also a black plastic tube that goes down below the cold coolant level. If you remove the vent line at the radiator and blow in to it, bubbles come out from that tube. It's just odd that there's two ports for that vent line; one above the coolant level and one below.

[LowR3V'in]

that's where the air from the radiator comes out.

[AWDBOB]

I don't have a late model expansion tank anymore, but that weep hole is present on quite a few other BMW models- it's how they "self bleed".

[CorvallisBMW]

I understand that description. Does the vent line from the radiator really dump into the air of the coolant reservoir?

A reservoir that dumped the rad vent below the coolant level would also solve that issue.

Capping the line might be good idea to try to see if that is what is happening.

The line actually seems to split. There is a small weep hole that bleeds above the coolant level. With the engine running and the cap off you can clearly see coolant coming out of it. But there's also a tube that goes down below the coolant level. If I blow air through the line, I can both hear it going through the weep hole and create bubbles from the bottom of the tube. It's quite odd.

Maybe that weep hole isn't supposed to be there? Anyone else with the same expansion tank able to confirm by running there engine with the cap off and seeing if coolant sprays in from a small hole in the neck?

edit: you can kind of see it here:
Capture2 by Darin Schnoor, on Flickr

[LJ851]

So here's my thinking on this: It's not that the air is going down through anywhere, it's that the coolant is pushing up. If there's 1.5" of fluid in the thermostat above the fluid in the expansion tank, that fluid is going to exert a downward force (called head pressure). It will push, albeit with a fairly small force, through the coolant passages of the engine and in to the expansion tank until the levels equalize. Meanwhile the air that's trapped in the expansion tank is going to push backwards through the vent line and in to the top of the radiator (this is what's different between my system and your bike; your reservoir is sealed, so the air cannot go anywhere. In mine, that vent line allows the air to move, which in turn allows the fluid to move. It's a straw with a finger only partially covering the top. If you drilled a tiny hole in your reservoir cap, the fluid would backflow down the line. Maybe I need to cap off my vent line???). Now with the engine off all of the pressures are equalized and the only force acting on the system is gravity. As such the fluid level is going to try and equalize itself everywhere in the system. Therefore the only way to keep air out of the engine is to ensure the fluid level in the expansion tank is at least as high as that point or higher.

I understand that description. Does the vent line from the radiator really dump into the air of the coolant reservoir?

A reservoir that dumped the rad vent below the coolant level would also solve that issue.

Capping the line might be good idea to try to see if that is what is happening.

[CorvallisBMW]

Once the air is fully gone its all fluid inside the cooling system, the only air is at the top of the expansion tank. That air is not going to go down through coolant in the tank and into the engine to equalize levels.

As an example, on one of my motorcycles i have the remote brake fluid reservoir completely below the clutch and brake master cylinders it feeds. The air on top of the fluid in the reservoir does not go down through the fluid and back up to the master cylinders and the fluid does not drain out of the master cylinders into the reservoir. Think a straw full of fluid with your finger on top of it.


I think your system is not fully bled or creating new air, or has a leak somewhere .

So here's my thinking on this: It's not that the air is going down through anywhere, it's that the coolant is pushing up. If there's 1.5" of fluid in the thermostat above the fluid in the expansion tank, that fluid is going to exert a downward force (called head pressure). It will push, albeit with a fairly small force, through the coolant passages of the engine and in to the expansion tank until the levels equalize. Meanwhile the air that's trapped in the expansion tank is going to push backwards through the vent line and in to the top of the radiator (this is what's different between my system and your bike; your reservoir is sealed, so the air cannot go anywhere. In mine, that vent line allows the air to move, which in turn allows the fluid to move. It's a straw with a finger only partially covering the top. If you drilled a tiny hole in your reservoir cap, the fluid would backflow down the line. Maybe I need to cap off my vent line???). Now with the engine off all of the pressures are equalized and the only force acting on the system is gravity. As such the fluid level is going to try and equalize itself everywhere in the system. Therefore the only way to keep air out of the engine is to ensure the fluid level in the expansion tank is at least as high as that point or higher.

Edit: Here's a simplified version of the diagram. The fluid levels will always equalize as long as the pressure is allowed to flow from one container to another. If I raise the expansion tank, the fluid level in it will drop as it moves to the engine. But as soon as it goes back to it's normal position, everything will level out. However if I were to block the vent line, that would prevent the pressures from equalizing, and then the fluid could be kept at dissimilar levels.
Capture1 by Darin Schnoor, on Flickr

[LJ851]

How does it not matter? Fluids always flow to the lowest point. And in a system with equalized pressures, the fluid levels would always equalize, right?

Yes I always bleed with the heater on high and the key in pos. 2

Once the air is fully gone its all fluid inside the cooling system, the only air is at the top of the expansion tank. That air is not going to go down through coolant in the tank and into the engine to equalize levels.

As an example, on one of my motorcycles i have the remote brake fluid reservoir completely below the clutch and brake master cylinders it feeds. The air on top of the fluid in the reservoir does not go down through the fluid and back up to the master cylinders and the fluid does not drain out of the master cylinders into the reservoir. Think a straw full of fluid with your finger on top of it.


I think your system is not fully bled or creating new air, or has a leak somewhere .

[CorvallisBMW]

Sometimes, what I've done, is remove the hose to the expansion tank to radiator (at the tank), place it in a container and fill the cooling system (overflowing, obviously) and allow whatever air/aeration to pass. M42 radiators usually require something similar to this, as they are a real pain in the ass to bleed out.

OK, I'm trying to picture this. The vent line is running from the rad to a container and the engine is running and the coolant is overflowing from the open expansion tank cap, yes? What makes this different than leaving the vent line attached to the tank and letting it purge in to there?

I'm sure you fill your cooling system through the upper hose as well, yes?

I haven't tried this method in a long time, as it only seems to fill the radiator. If the thermostat is closed coolant can't move through and in to the engine, yes? Lately I've been filling via a vacuum filler like this, which is supposedly the penultimate method: https://www.ecstuning.com/b-schwaben.../024948sch01a/

[e30austin]

Sometimes, what I've done, is remove the hose to the expansion tank to radiator (at the tank), place it in a container and fill the cooling system (overflowing, obviously) and allow whatever air/aeration to pass. M42 radiators usually require something similar to this, as they are a real pain in the ass to bleed out. I'm sure you fill your cooling system through the upper hose as well, yes?

[CorvallisBMW]

Once a cooling system is bled, the location of the components doesn't really matter.

Are you bleeding the system with the heater fully on (valve open) ?

How does it not matter? Fluids always flow to the lowest point. And in a system with equalized pressures, the fluid levels would always equalize, right?

Yes I always bleed with the heater on high and the key in pos. 2

so when it over heats just let it cool down and crank the bleed screw
and fill up. Even when i blew my head gasket i just put a gal back in and drove home without it overheating.
bend and wiggle the little hose going to the tank while filling it up i think there is something just clogged or backwards

Yes, I've opened the bleed screw to purge the air literally dozens of times. I always get air out, then I drive for another 10-20 miles, and get more air. I can do this ad nasuem from now until eternity. I've replaced the vent line with 100% new hose and verified it flows correctly. But as I've said above, the vent line is BELOW the thermostat housing. So if air gets trapped in the thermostat housing, the vent line won't purge it.