BMW Expansion Tanks, Caps, and Bleeding Explained

[R Shaffner]

If your BMW has a coolant cap that looks like this (with two O-Rings and a washer), then this thread is for you.
Attachment 641300

And if your BMW has an expansion tank that is mounted as part of or beside the radiator, as in the examples below, then this thread is for you.

Attachment 641301

We can probably all agree that these cooling systems are more complicated than others we've dealt with. Many of us would call them over-engineered. We have to fill them differently. And when we do things to them they can behave in unexpected ways. (For example, if you loosen the bleed screw on a cold properly-bled engine, the coolant level in the tank will magically rise.)

I don't have all the answers. Not close. I'm starting this thread because I'm curious and I know all of us are smarter than some of us. I hope others will weigh-in over time and this thread will eventually provide most if not all the answers.

Also, I have some images and insights to share that I haven't found elsewhere on this site, that will be new to some readers here. I'll start with those in the next post.

[R Shaffner]

I think that's why BMW went through all this. It's about space and packaging.

It's easy to put an unpressurized expansion tank at the same level as the radiator, or above or below it. And it's easy to put a pressurized tank above the level of the radiator. But things get tricky when a manufacturer wants to put the expansion tank beside the radiator, at the same level. Generally that means the level of the coolant in the tank must be blow the level of the coolant in the full radiator.

Following is the best diagram I found for this after a fair amount of Googling. This came from an AutoZone page and it's supposedly for an e36. (I'm happy to be corrected or use a better diagram is anyone has one.)

Attachment 641302

First, I should point out that I think there's a big omission in this diagram -- the bleed screw is missing. I'm thinking it should be shown along the top of the radiator, near the top of tube 3, where it would let air out. That's consistent with other pictures. Other than that mistake I think this diagram is pretty good for this discussion.

Here's how the system would work, filling it from empty and then running it.

Let's start with the bleed screw closed. As coolant is poured into the open Cap 1, in flows into the tank and out Tube 6 to the water pump. BMW re-fill procedures call for getting the coolant to flow through the engine and the heater core with the secondary pump (and/or starting the engine, as I like to do it).

I'm not sure about all models, but eventually the engine, head, heater cores and other coolant passages all get filled until coolant comes back to the radiator through tubes 2 and 4, and then the radiator core starts filling up.

Here it's easy to see how air would get trapped at the top of the radiator core. Imagine what would happen if the expansion tank and the radiator were both half full. Pouring more coolant in the tank would not raise the level in the radiator unless the trapped air had a way to get out. That's why BMW uses a bleed screw in cases like this. It's the fastest, easiest way to get the radiator full of coolant.

The Bentley manual for my car (2002 540i) says to open the bleed screw and fill the system all the way up. Then to put the cap on, replace the bleed screw, and run the car. As the coolant expands excess coolant will be blown out the cap. When the engine cools air will be sucked back into the top of the tank and the coolant should be at the right level. That works, of course, but so do many other ways for refilling and bleeding the system.

Here's why: To some extent this system is self-bleeding, thanks to the expansion and contraction of the coolant as the engine heats up and cools down.

Look at the diagram above and imagine about 1/2 inch of air in the top of the radiator when it's cold. As the coolant heats up and expands, some of that air will be pushed out of the radiator through Tube 3, where it goes to the bottom of the expansion tank and floats to the top. When the coolant cools down, it contracts and some coolant is sucked from the bottom of the expansion tank, back up Tube 3, to the top of the radiator.

If the owner were to open the cap the next morning, he'd see less coolant in the expansion tank and he might need to add more, to make up for the fact that the radiator wasn't as full of coolant before. (Some users here say they never use the bleed screw; they just monitor everything and add coolant as needed over the next several cycles, as the system self-bleeds.)

There's more to discuss with this diagram, including the cap and the "control" line 5, but this is long enough for this post. More to come later, unless others beat me to it.

[Breeze1]

Interesting post, thanks. The diagram is not necessarily wrong about the bleed screw: in my car it's located on an elbow that comes out of the side of the tank and goes to the engine.

[R Shaffner]

In the last post I showed a diagram, Now I'd like to move on to a concrete example. This is for the M62 and M62TU, one of the more common V8s used in the 740i and 540i, among others. It's also the example I know the best.

First, here is a cut-away picture of the tank for the M62TU (thanks to JimLev). I believe the tank for the M62 is the same.

Attachment 641306

And here is a diagram JimLev prepared, which I modified slightly. In this case, the expansion tank is mounted beside the radiator and connected to it with hoses.

Attachment 641307

As in the diagram for the e36, the expansion tank feeds the water pump from a hose at the bottom. The tank has a float with a red stick to indicate the coolant level. (When the top of the sick is level with the top of the tank, with cold coolant, it's full.)

On the top left of the diagram above is a fitting for a small hose that comes from the top of the radiator, just above the top radiator hose from the engine. Air and coolant from this hose flow down half of a walled white tube to the bottom of the expansion tank, just like Tube 3 in the previous example. When the radiator is being filled, it can be bled using the bleed screw. Air remaining in the radiator will be self-bled as air is forced by the heat/expansion cycle down the white tube and into the expansion tank.

This diagram explains a lot. Again, it explains why BMW added the bleed screw. If one simply filled the expansion tank without opening the bleed screw and circulating the coolant (i.e., like filling the system on a normal car), then the top part of the radiator would remain unfilled.

It also explains why the radiator can be full of coolant, and that coolant remains at that level (higher than the level in the expansion tank), even when the cap of the tank is removed. Because that pathway from the hose goes down the side of the white tube and opens up under the coolant level, so there's no way for air to get inside the radiator and let the coolant out.

And it explains why the level of coolant in the expansion tank rises when I unscrew the bleed screw. (On a cold engine with the cap removed, the float and red stick will rise when one loosens the bleed screw.) I believe that lets air past the screw into the top of the radiator. That lets some coolant flow backwards in the system, back into the expansion tank.

When I did that on my car recently the red stick went from 3" below the top of the expansion tank to 2" above. I put the cap back on with it that way, drove it around, got the car up to operating temp, and parked it. When I went out the next day the red stick had dropped down again, to the -3 inch level. And once again when I loosened the bleed screw it came back up.

So, even though the bleed screw helps when re-filling, the system is designed to be somewhat self-bleeding. In other words, when one is driving around any air in the system will end up along the top of the radiator. When the coolant gets hot and expands, that air is forced along the tube, past the bleed valve and down into the expansion tank. If there's enough air to get forced out of the white tube at the bottom of the tank, it will leave that pathway and float up to the top of the tank. Any air that doesn't make it out of the bottom of the white expansion tube will stay the tube and/or the hose, expanding and contracting and not hurting anything, unless that little bit of air gets bled out too.

And finally, that explains why all these different methods for bleeding the system work. All we really need to do is get it close, making sure there's enough coolant to work, and then check it again after it cools. Over time the system will force out almost all of the air the we might leave in the wrong place.

So that's the example using the M62. Next I'll discuss the cap and ask some questions about the other half of the white tube in the M62, which corresponds to the Control Line in the e36 example.

- - - Updated - - -

Interesting post, thanks. The diagram is not necessarily wrong about the bleed screw: in my car it's located on an elbow that comes out of the side of the tank and goes to the engine.

Great. That's good to know. Thanks!

[R Shaffner]

The short answer is "I'm not sure." The longer answer starts below.

First, let me start with a picture of the outside of the cap. Notice the 2 O-rings on the barrel part of the cap, and the flat rubber washer that seals the tank. The yellow arrow points to a hole that is between the O-rings, as discussed more below.

Attachment 641314

I have a spare 540i parts car and I decided to cut open the coolant cap since I couldn't find a picture or diagram anywhere. I was also curious about why the cap had 2 O-rings that fit into the throat of the tank. (I apologize in advance for the quality of the pictures. The parts are almost all black so it was hard for me to get a good shot of them.)

Here is a picture of the cap from an M62 (same as M62TU, and I believe it's similar to those for many other BMWs). The parts on the right were pried out of the cap on the left. The assembly with the spring and the pressure-relief valve is on the top, not in cross-section. Included in that assembly is the valve that lets air back into the tank when things cool, as needed.

The bottom half of the parts on the right are cut-away. That includes the rubber washer and the two O-rings. It also includes the parts that presumably have something to do with the "control line" shown in the e36 diagram.
Attachment 641312

If I'd made a different cut of this cap, rotated 90 degrees, you'd be able to see passages that let air/coolant go from the tank below the cap to the pressure-relief assembly in the top half of the cap. So the top of the assembly, with the 2 valves and the spring, works like any regular coolant cap. When the pressure gets above a certain level the valve opens to let air/coolant out. If the cold coolant contracts enough another valve opens to let air back in.

What's odd about these caps is the part in the middle. Here I made the cut to bisect the hole that the yellow arrow points to in the first picture. If you look closely, on the right side of the lower assembly, you can see a whitish horizontal passage way that is the inside of half of that hole.

So, this hole is at the top of the other half of the walled white tube from in the M62TU diagram. That corresponds to the the Control Line 6 in the e36 diagram. From what I can tell, that hole leads to a closed chamber in the middle of the cap that has a rubber diaphragm along the top. In other words, it's not a vent. With the cap on, the O-Rings seal this passage. So the control line takes pressure from the bottom of the tank to the middle of the cap. What I don't know is why.

It looks like it could be a way to help the the pressure-relief valve open. Pressure would come up the control line tube, into the center of the cap, pushing up on the rubber diaphragm, which would push up on a little piston above it, which looks like it might exert upward pressure on the parts above it, which would open the valve. Maybe.

But what a long way around the block that would be. Pressure is pressure. Other radiator caps just use air or coolant pressure to open the valve. No special diaphragms or pistons are needed.

Up until now I've been offering my thoughts and inviting reactions/corrections. Now I really don't know and I'm asking.

Also, it's worth noting that the e36 diagram shows some kind of float or check valve in the control line passage, just below where it joins the cap. I cut open that part of my M62 tank and it was just an open passage way.

Does anyone know how this control line and this part of the tank cap are supposed to work? And why would BMW do it this way?

[R Shaffner]

Here are more cap pictures that might help. First here's a close up of the "control" part of the cap, if thats' what it really is. (Ignore the O-rings in this picture. The top one is in the wrong place.)
Attachment 641315

And here's a picture from a different perspective.

Attachment 641316

[R Shaffner]

And now that I've posted this, I have to wonder why BMW didn't just use a non-pressurized coolant recovery tank and a normal radiator cap. The could have put it at the same level as the current expansion tanks -- no problem. Far easier, cheaper and more reliable.

Someone probably asked an engineer if the tank could be put lower. The answer was the German word for "sure." Not worth the effort in my opinion.

[shadowpuck]

nice series of posts - thank you for taking the time to post this.

it has always seemed complicated to me - but, having a few friends in the auto industry i've learned that "everything has a reason" even if the reason is kinda crazy! on many modern vehicles you can start with two arguments for any engineering decision "cost/production impact or efficiency" or "fuel efficiency." the omnipresent "required by regulation" is the overall champion.....

[shogun]

copied from general forum to E32 forum, and my info to that here https://www.bimmerforums.com/forum/s...5#post30206385

[jclausen]

I remember those plastic bleed lines from the expansion tank to the radiator causing issues when clogged

[shogun]

yes, also on the E32 a problem, Gale has that explained here with pics http://www.nmia.com/~dgnrg/page_19.htm
I replaced the old thin original plastic pipe on my 11/88 750 between radiator and expansion tank with a fitting fuel hose with a bit larger dia and without so many bends.